gdb: Add debug tracing for bfd cache activity.

gdb: Add debug tracing for bfd cache activity.

This patch adds a new debug flag bfd-cache, which when set to non-zero
produces debugging log messages relating to gdb's bfd cache.

gdb/ChangeLog:

	* gdb_bfd.c (debug_bfd_cache): New variable.
	(show_bfd_cache_debug): New function.
	(gdb_bfd_open): Add debug logging.
	(gdb_bfd_ref): Likewise.
	(gdb_bfd_unref): Likewise.
	(_initialize_gdb_bfd): Add new set/show command.
	* NEWS: Mention new command.

gdb/doc/ChangeLog:

	* gdb.texinfo (File Caching): Document "set/show debug bfd-cache".