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
standard 2-clause FreeBSD license for those files where I have obtained
permission from all of the listed copyright holders.

Approved by: jkoshy, kaiw
Differential Revision: https://reviews.freebsd.org/D14561