Make extract_arg return a std::string

Change extract_arg to return a std::string and fix up all the users.
I think string is mildly better than unique_xmalloc_ptr<char>, when
possible, because it provides a more robust API.

I changed the error messages emitted from find_location_by_number to
avoid either writing to a string or an extra allocation; this can be
changed but I thought that the new message was not any less clear.
You can see an example in the testsuite patch.

ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* demangle.c (demangle_command): Update.
	* breakpoint.c (disable_command): Update.
	(enable_command): Update.
	(find_location_by_number): Make "number" const.  Use
	get_number_trailer.
	* cli/cli-utils.c (extract_arg): Return std::string.
	* probe.c (parse_probe_linespec): Update.  Change types.
	(collect_probes): Take string arguments.
	(parse_probe_linespec): Likewise.
	(info_probes_for_ops): Update.
	(enable_probes_command): Update.
	(disable_probes_command): Update.
	* break-catch-sig.c (catch_signal_split_args): Update.
	* mi/mi-parse.c (mi_parse): Update.

testsuite/ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* gdb.base/ena-dis-br.exp (test_ena_dis_br): Update test.

1 files changed, 3 insertions, 9 deletions

diff --git a/gdb/mi/mi-parse.c b/gdb/mi/mi-parse.c
index ad1478e..cf05fa0 100644
--- a/gdb/mi/mi-parse.c
+++ b/gdb/mi/mi-parse.c
@@ -347,20 +347,14 @@ mi_parse (const char *cmd, char **token)
 	}
       else if (strncmp (chp, "--language ", ls) == 0)
 	{
-	  char *lang_name;
-	  struct cleanup *old_chain;
-
 	  option = "--language";
 	  chp += ls;
-	  lang_name = extract_arg (&chp);
-	  old_chain = make_cleanup (xfree, lang_name);
+	  std::string lang_name = extract_arg (&chp);
 
-	  parse->language = language_enum (lang_name);
+	  parse->language = language_enum (lang_name.c_str ());
 	  if (parse->language == language_unknown
 	      || parse->language == language_auto)
-	    error (_("Invalid --language argument: %s"), lang_name);
-
-	  do_cleanups (old_chain);
+	    error (_("Invalid --language argument: %s"), lang_name.c_str ());
 	}
       else
 	break;