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.