Change program_space::ebfd to a gdb_bfd_ref_ptr

Change program_space::ebfd to a gdb_bfd_ref_ptr

This changes program_space::ebfd to a gdb_bfd_ref_ptr, removing some
manual management.

gdb/ChangeLog
2020-10-29  Tom Tromey  <tom@tromey.com>

	* exec.c (exec_file_attach): Update.
	* progspace.c (program_space::exec_close): Update.
	* progspace.h (struct program_space) <ebfd>: Now a
	gdb_bfd_ref_ptr.
	<set_exec_bfd>: Change argument type.
	<exec_bfd>: Update.