gdb: New maintenance command to disable bfd sharing.

gdb: New maintenance command to disable bfd sharing.

In some rare maintainer cases it is desirable to be able to disable bfd
sharing.  This patch adds new commands maintenance set/show commands for
bfd-sharing, allowing gdb's bfd cache to be turned off.

gdb/ChangeLog:

	* gdb_bfd.c (bfd_sharing): New variable.
	(show_bfd_sharing): New function.
	(gdb_bfd_open): Check bfd_sharing variable.
	(_initialize_gdb_bfd): Add new set/show command.
	* NEWS: Mention new command.

gdb/doc/ChangeLog:

	* gdb.texinfo (Maintenance Commands): Move documentation of "main
	info bfds" to...
	(File Caching): A New section.  Outline bfd caching, and add new
	description for "main set/show bfd-sharing".