Load system gdbinit files from a directory

Load system gdbinit files from a directory

Adds a configure option --with-system-gdbinit-dir to specify a directory
in which to look for gdbinit files.  All files in this directory are
loaded on startup (subject to -n/-nx as usual) as long as the extension
matches a known and enabled scripting language (.gdb/.py/.scm).

This also changes get_ext_lang_of_file to support ".gdb" files, similar
to get_ext_lang_defn's handling of EXT_LANG_GDB.

gdb/ChangeLog:

2019-10-29  Christian Biesinger  <cbiesinger@google.com>

	* NEWS: Mention new --with-system-gdbinit-dir option.
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add new option --with-system-gdbinit-dir.
	* extension.c (get_ext_lang_of_file): Return extension_language_gdb
	for a ".gdb" suffix.
	* main.c (get_init_files): Change system_gdbinit argument to
	a vector and return the files in SYSTEM_GDBINIT_DIR in
	addition to SYSTEM_GDBINIT.
	(captured_main_1): Update.
	(print_gdb_help): Update.
	* top.c (print_gdb_configuration): Also print the value of
	SYSTEM_GDBINIT_DIR.

gdb/doc/ChangeLog:

2019-10-29  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in: Also set SYSTEM_GDBINIT_DIR for the info manual
	generation.
	* gdb.texinfo (many sections): Document new --with-system-gdbinit-dir
	option.

Change-Id: If233859ecc21bc6421d589b37cd658a3c7d030f2