In r113596, version 4 of entries have been added but pwd_mkdb have
been generating both new (machine independent) and legacy version
entries (endianness sensitive).

The base system have been using the new format for quite some time,
so disable the generation by default.

An interim option, -l, have been added to re-enable old behavior.
The -l, -B and -L options are considered deprecated and will be
removed in FreeBSD 12.0 release.