gdb: process early initialization files and command line options

Adds the ability to process commands at a new phase during GDB's
startup.  This phase is earlier than the current initialisation file
processing, before GDB has produced any output.

The number of commands that can be processed at this early stage will
be limited, and it is expected that the only commands that would be
processed at this stage will relate to some of the fundamentals of how
GDB starts up.

Currently the only commands that it makes sense to add to this early
initialization file are those like 'set style version ....' as the
version string is displayed during startup before the standard
initialization files are parsed.  As such this commit fully resolved
bug cli/25956.

This commit adds a mechanism to execute these early initialization
files from a users HOME directory, as well as some corresponding
command line flags for GDB.

The early initialization files that GDB will currently check for are
~/.config/gdb/gdbearlyinit (on Linux like systems) or ~/.gdbearlyinit
if the former is not found.

The output of 'gdb --help' has been extended to include a list of the
early initialization files being processed.

gdb/ChangeLog:

	PR cli/25956
	* NEWS: Mention new early init files and command line options.
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Define GDBEARLYINIT.
	* main.c (get_earlyinit_files): New function.
	(enum cmdarg_kind): Add CMDARG_EARLYINIT_FILE and
	CMDARG_EARLYINIT_COMMAND.
	(captured_main_1): Add support for new command line flags, and for
	processing startup files.
	(print_gdb_help): Include startup files in the output.

gdb/doc/ChangeLog:

	PR cli/25956
	* gdb.texinfo (File Options): Mention new command line options.
	(Startup): Discuss when early init files are processed.
	(Initialization Files): Add description of early init files.
	(Output Styling): Update description of 'version' style.
	(gdb man): Mention early init files.

gdb/testsuite/ChangeLog:

	PR cli/25956
	* gdb.base/early-init-file.c: New file.
	* gdb.base/early-init-file.exp: New file.
	* lib/gdb-utils.exp (style): Handle style 'none'.

1 files changed, 12 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index 6cf76a1..c5f8605 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -48,6 +48,18 @@
   the use of the GNAT encoding (based on information added to the type's
   name following a GNAT-specific format).
 
+* GDB will now load and process commands from ~/.config/gdb/gdbearlyinit
+  or ~/.gdbearlyinit if these files are present.  These files are
+  processed earlier than any of the other initialization files and
+  can affect parts of GDB's startup that previously had already been
+  completed before the initialization files were read, for example
+  styling of the initial GDB greeting.
+
+* GDB now has two new options "--early-init-command" and
+  "--early-init-eval-command" with corresponding short options "-eix"
+  and "-eiex" that allow options (that would normally appear in a
+  gdbearlyinit file) to be passed on the command line.
+
 * New commands
 
 set debug event-loop