Introduce new_bfd_ref

Introduce new_bfd_ref

This introduces a helper function, new_bfd_ref, that calls gdb_bfd_ref
and returns a gdb_bfd_ref_ptr.  Then it updates several places to use
this.

ChangeLog
2017-10-20  Tom Tromey  <tom@tromey.com>

	* exec.c (exec_file_attach): Use new_bfd_ref.
	* symfile-mem.c (symbol_file_add_from_memory): Use new_bfd_ref.
	* gdb_bfd.c (gdb_bfd_open, gdb_bfd_fopen, gdb_bfd_openr)
	(gdb_bfd_openw, gdb_bfd_openr_iovec, gdb_bfd_fdopenr): Use
	new_bfd_ref.
	* gdb_bfd.h (new_bfd_ref): New function.