Add support for the readnever concept

The purpose of this concept is to turn the load of debugging
information off, either globally (via the '--readnever' option), or
objfile-specific.  The implementation proposed here is an extension of
the patch distributed with Fedora GDB; looking at the Fedora patch
itself and the history, one can see some reasons why it was never
resubmitted:

  - The patch appears to have been introduced as a workaround, at
    least initially;
  - The patch is far from perfect, as it simply shunts the load of
    DWARF debugging information, without really worrying about the
    other debug format.
  - Who really does non-symbolic debugging anyways?

One use of this feature is when a user simply wants to do the
following sequence: attach, dump core, detach.  Loading the debugging
information in this case is an unnecessary cause of delay.

This patch expands the version shipped with Fedora GDB in order to
make the feature available for all the debuginfo backends, not only
for DWARF.  It also implements a per-objfile flag which can be
activated by using the "-readnever" command when using the
'add-symbol-file' or 'symbol-file' commands.

It's also worth mentioning that this patch tests whether GDB correctly
fails to initialize if both '--readnow' and '--readnever' options are
passed.

Tested on the BuildBot.

gdb/ChangeLog:

2017-12-01  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0: Mention new '--readnever'
	feature.
	* coffread.c (coff_symfile_read): Do not map over sections with
	'coff_locate_sections' if readnever is on.
	* dwarf2read.c (dwarf2_has_info): Return 0 if
	readnever is on.
	* elfread.c (elf_symfile_read): Do not map over sections with
	'elf_locate_sections' if readnever is on.
	* main.c (validate_readnow_readnever): New function.
	(captured_main_1): Add support for --readnever.
	(print_gdb_help): Document --readnever.
	* objfile-flags.h (enum objfile_flag) <OBJF_READNEVER>: New
	flag.
	* symfile.c (readnever_symbol_files): New global.
	(symbol_file_add_with_addrs): Set 'OBJF_READNEVER' when
	'READNEVER_SYMBOL_FILES' is set.
	(validate_readnow_readnever): New function.
	(symbol_file_command): Handle '-readnever' option.
	Call 'validate_readnow_readnever'.
	(add_symbol_file_command): Handle '-readnever' option.
	Call 'validate_readnow_readnever'.
	(_initialize_symfile): Document new '-readnever' option for
	both 'symbol-file' and 'add-symbol-file' commands.
	* top.h (readnever_symbol_files): New extern global.
	* xcoffread.c (xcoff_initial_scan): Do not read debug
	information if readnever is on.

gdb/doc/ChangeLog:

2017-12-01  Andrew Cagney  <cagney@redhat.com>
	    Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (File Options): Document --readnever.
	(Commands to Specify Files): Likewise, for 'symbol-file' and
	'add-symbol-file'.

gdb/testsuite/ChangeLog:

2017-12-01  Joel Brobecker  <brobecker@adacore.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.base/readnever.c, gdb.base/readnever.exp: New files.

1 files changed, 34 insertions, 3 deletions

diff --git a/gdb/main.c b/gdb/main.c
index 61168fa..8f04da1 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -402,6 +402,19 @@ symbol_file_add_main_adapter (const char *arg, int from_tty)
   symbol_file_add_main (arg, add_flags);
 }
 
+/* Perform validation of the '--readnow' and '--readnever' flags.  */
+
+static void
+validate_readnow_readnever ()
+{
+  if (readnever_symbol_files && readnow_symbol_files)
+    {
+      error (_("%s: '--readnow' and '--readnever' cannot be "
+	       "specified simultaneously"),
+	     gdb_program_name);
+    }
+}
+
 /* Type of this option.  */
 enum cmdarg_kind
 {
@@ -579,14 +592,17 @@ captured_main_1 (struct captured_main_args *context)
       OPT_NOWINDOWS,
       OPT_WINDOWS,
       OPT_IX,
-      OPT_IEX
+      OPT_IEX,
+      OPT_READNOW,
+      OPT_READNEVER
     };
     static struct option long_options[] =
     {
       {"tui", no_argument, 0, OPT_TUI},
       {"dbx", no_argument, &dbx_commands, 1},
-      {"readnow", no_argument, &readnow_symbol_files, 1},
-      {"r", no_argument, &readnow_symbol_files, 1},
+      {"readnow", no_argument, NULL, OPT_READNOW},
+      {"readnever", no_argument, NULL, OPT_READNEVER},
+      {"r", no_argument, NULL, OPT_READNOW},
       {"quiet", no_argument, &quiet, 1},
       {"q", no_argument, &quiet, 1},
       {"silent", no_argument, &quiet, 1},
@@ -809,6 +825,20 @@ captured_main_1 (struct captured_main_args *context)
 	    }
 	    break;
 
+	  case OPT_READNOW:
+	    {
+	      readnow_symbol_files = 1;
+	      validate_readnow_readnever ();
+	    }
+	    break;
+
+	  case OPT_READNEVER:
+	    {
+	      readnever_symbol_files = 1;
+	      validate_readnow_readnever ();
+	    }
+	    break;
+
 	  case '?':
 	    error (_("Use `%s --help' for a complete list of options."),
 		   gdb_program_name);
@@ -1183,6 +1213,7 @@ Selection of debuggee and its files:\n\n\
   --se=FILE          Use FILE as symbol file and executable file.\n\
   --symbols=SYMFILE  Read symbols from SYMFILE.\n\
   --readnow          Fully read symbol files on first access.\n\
+  --readnever        Do not read symbol files.\n\
   --write            Set writing into executable and core files.\n\n\
 "), stream);
   fputs_unfiltered (_("\