ar: Disallow directory traversal

Set ARCHIVE_EXTRACT_SECURE_SYMLINKS and ARCHIVE_EXTRACT_SECURE_NODOTDOT
as in bsdtar to prevent extraction of archive entries whose pathnames
contain .. or whose target directory would be altered by a symlink.
Also disallow absolute pathnames.

We don't currently provide an option to disable this behaviour (as
bsdtar's -P does). It is unlikely to be a problem in practice for ar(1),
but the -P option is not currently used and available if we want to
consider it for this purpose.

Differential Revision: https://reviews.freebsd.org/D1524
Reported by: Alexander Cherepanov <cherepan@mccme.ru>
Approved by: delphij
Obtained from: ELF tool chain ar, Ticket #474
MFC after: 1 week
Relnotes: Yes
Sponsored by: The FreeBSD Foundation