usr.bin/ar: use standard 2-Clause FreeBSD license

Many licenses on ar files contained small variations from the standard
FreeBSD license text. To avoid license proliferation switch to the usual
2-clause FreeBSD license after obtaining permission from all copyright
holders.

Approved by: jkoshy, kaiw, kientzle
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14561