Replace remaining cleanups in fbsd-nat.c.

Replace remaining cleanups in fbsd-nat.c.

- Use a custom deleter with std::unique_ptr to free() memory returned
  by kinfo_getvmmap().
- Use std::string with string_printf() to generate the pathname of the
  procfs 'map' file.
- Use gdb::byte_vector to manage the dynamic buffer for
  TARGET_OBJECT_AUXV and the dynamically allocated array of LWP IDs.

gdb/ChangeLog:

	* fbsd-nat.c [HAVE_KINFO_GETVMMAP] (struct free_deleter): New.
	(fbsd_find_memory_regions): Use free_deleter with std::unique_ptr.
	[!HAVE_KINFO_GETVMMAP] (fbsd_find_memory_regions): Use std::string
	for `mapfilename'.
	(fbsd_xfer_partial): Use gdb::byte_vector.
	(fbsd_add_threads): Likewise.