Add add-auto-load-scripts-directory.

Add add-auto-load-scripts-directory.

There is already "add-auto-load-safe-path" which works
like "set auto-load safe-path" but in append mode.

There was missing an append equivalent for "set auto-load scripts-directory".

ABRT has directory /var/cache/abrt-di/ as an alternative one
to /usr/lib/debug/ . Therefore ABRT needs to use -iex parameters to add this
/var/cache/abrt-di/ directory as a first-class debuginfo directory.
Using absolute "set auto-load scripts-directory" would hard-code the path
possibly overriding local system directory additions; besides it would not be
nice anyway.

gdb/ChangeLog
2014-11-30  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Add add-auto-load-scripts-directory.
	* NEWS (Changes since GDB 7.8): Add add-auto-load-scripts-directory.
	* auto-load.c (add_auto_load_dir): New function.
	(_initialize_auto_load): Install it.

gdb/doc/ChangeLog
2014-11-30  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Add add-auto-load-scripts-directory.
	* gdb.texinfo (Auto-loading): Add add-auto-load-scripts-directory link.
	(objfile-gdbdotext file): Add add-auto-load-scripts-directory.