[binutils-gdb] Introduce and use gdb::unlinker

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT bef155c3e8a995fcdb1c2ba5aba012eb653d9f30 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: bef155c3e8a995fcdb1c2ba5aba012eb653d9f30

Introduce and use gdb::unlinker

This introduces a new class, gdb::unlinker, that unlinks a file in the
destructor.  The user of this class has the option to preserve the
file instead, by calling the "keep" method.

This patch then changes the spots in gdb that use unlink in a cleanup
to use this class instead.  In one spot I went ahead and removed all
the cleanups from the function.

This fixes one latent bug -- do_bfd_delete_cleanup could refer to
freed memory, by decref'ing the BFD before using its filename.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* record-full.c (record_full_save_cleanups): Remove.
	(record_full_save): Use gdb::unlinker.
	* gcore.c (do_bfd_delete_cleanup): Remove.
	(gcore_command): Use gdb::unlinker, unique_xmalloc_ptr.  Remove
	cleanups.
	* dwarf2read.c (unlink_if_set): Remove.
	(write_psymtabs_to_index): Use gdb::unlinker.
	* common/gdb_unlinker.h: New file.