Constify main.c:get_init_files.

Constify main.c:get_init_files.

A following patch will want to make the "gdbinit" global array const.
As usual, that forces in a cascading series of const additions.  This
patch preemptively does those.  I went all the way up to constifying
catch_command_errors, but then that would require constifying
execute_command as well (which is a much more significant effort).  So
as stop point, I found the cleanest would be to add a variant of
catch_command_errors that takes const args, and use that in the few
spots that needed it due to the the get_init_files constification.

gdb/
2013-07-01  Pedro Alves  <palves@redhat.com>

	* cli/cli-cmds.c (source_script): Make 'file' parameter const.
	* cli/cli-cmds.h (source_script): Likewise.
	* exceptions.c (catch_command_errors_const): New function.
	* exceptions.h (catch_command_errors_const): Declare.
	* main.c (get_init_files): Make parameters const, and adjust.
	(captured_main): Make 'system_gdbinit', 'home_gdbinit' and
	'local_gdbinit' locals const.  Adjust to use
	catch_command_errors_const.
	(print_gdb_help): Make 'system_gdbinit', 'home_gdbinit' and
	'local_gdbinit' locals const.