Use gdb::unique_xmalloc_ptr<> instead of a deleter that invokes free().

Use gdb::unique_xmalloc_ptr<> instead of a deleter that invokes free().

Since xfree() always wraps free(), it is safe to use the xfree deleter
for buffers allocated by library routines such as kinfo_getvmmap() that
must be released via free().

gdb/ChangeLog:

	* fbsd-nat.c (struct free_deleter): Remove.
	(fbsd_find_memory_regions): Use gdb::unique_xmalloc_ptr<>.