Add tar(1) -F option to select write format

Add tar(1) -F option to select write format

We want to move towards 'pax' as the default format for writing, this
option lets users downgrade to -F ustar where the 'pax' format isn't
convenient/usable (same as -x <format> in pax(1)).

-F <format> is more generic than -o/-O. -H (GNU tar) was already used
and we don't want long options so --format (NetBSD/FreeBSD) is excluded
too.

ok sthen@ caspar@ millert@