pwd_mkdb: add legacy support deprecation notice

MFC r332789: pwd_mkdb: warn that legacy support is deprecated (if specified)

r283981 switched pwd_mkdb to emit only v4 database entries by default,
and introduced a -l (legacy) option emit v3 entries in addition. The
commit message claims that legacy support will be removed in 12.0, so
emit a warning now if it is used.

MFC r332875: pwd_mkdb: add deprecation notice in manpage too

Followon to r332789; as reported on the -current and -stable lists and
in review D15144 the -l option will be removed before FreeBSD 12.0.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation