Add support for option "-M", which is used to operate ar(1) in a
script mode like the MRI(Microtec Research Inc.) "librarian" program.

Originally this option is provided by Binutils ar(1) to ease the
transition for developers who are used to writing "librarian" scripts.

We added this option to BSD ar(1) because:

1. Further improve the compatibility with Binutils ar(1).
2. There are still a few software using this -M option. (at least one

in our ports collection)

Suggested by: rink & erwin