Make "cbfd" a gdb_bfd_ref_ptr

This changes program_space::cbfd to be a gdb_bfd_ref_ptr.  This makes
it somewhat less error-prone to use, because now it manages the
reference counting automatically.

Tested by the buildbot.

2018-05-16  Tom Tromey  <tom@tromey.com>

	* gdbcore.h (core_bfd): Redefine.
	* corelow.c (core_target::close): Update.
	(core_target_open): Update.
	* progspace.h (struct program_space) <cbfd>: Now a
	gdb_bfd_ref_ptr.