gdb: bool-ify ext_lang_auto_load_enabled and friends

gdb: bool-ify ext_lang_auto_load_enabled and friends

Make it and related functions return bool.  Move function comments to
header where applicable.

gdb/ChangeLog:

	* auto-load.h (auto_load_gdb_scripts_enabled): Return bool, move
	comment here.
	* auto-load.c (auto_load_gdb_scripts_enabled): Return bool, move
	comment to header.
	* extension-priv.h (struct extension_language_script_ops)
	<auto_load_enabled>: Return bool.
	* extension.h (ext_lang_auto_load_enabled): Return bool, move
	comment here.
	* extension.c (ext_lang_auto_load_enabled): Return bool, move
	comment to header.
	* guile/guile-header.h (gdbscm_auto_load_enabled): Return bool,
	move comment here.
	* guile/scm-auto-load.c (gdbscm_auto_load_enabled): Return bool,
	move comment to header.
	* python/python-header.h (gdbpy_auto_load_enabled): Return bool,
	move comment here.
	* python/py-auto-load.c (gdbpy_auto_load_enabled): Return bool,
	move comment to header.

Change-Id: I657a17d2dab77a36884a137ce9b23a2cc6d53140