pwd_mkdb: retire legacy v3 db support (-l option)

pwd_mkdb has emitted v4 password database records since 2003 (r113596)
in addition to v3, and as of r283981 by default it emitted only v4.
As described in r283981, retire the -l legacy option.

The -B and -L options were originally added to set the endianness of v3
records emitted by pwd_mkdb, but they also set the db hash endiannes and
so have been retained temporarily.

Announced on the FreeBSD-Current and FreeBSD-Stable lists. In stable/11
the man page contains a deprecation notice, and pwd_mkdb will emit a
deprecation notice if the -l option is specified.

Reviewed by: delphij, lidl, rgrimes
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15144