ar: enable deterministic mode by default

Ar cannot handle UIDs with more than 6 digits, and storing the mtime,
uid, gid and mode provides little to negative value anyhow for ar's
uses. Turn on deterministic (-D) mode by default; it can be disabled by
the user with -U.

PR: 196929
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3190