Issue a STYLE message when normalizing the date format in .Dd/.TH.

Issue a STYLE message when normalizing the date format in .Dd/.TH.
Leah Neukirchen pointed out that mdoclint(1) used to warn about a
leading zero before the day number, so we know that both NetBSD and
Void Linux want the message.  It does no harm on OpenBSD because
Mdocdate always does the right thing anyway.
jmc@ agrees that it makes sense in contexts not using Mdocdate.