Another 18% speedup for mandocdb(8) -Q, found by gprof(1).

Another 18% speedup for mandocdb(8) -Q, found by gprof(1).
In -Q mode, refrain form validating and normalizing the format
of the date given in .Dd or .TH, as it won't be used anyway.

For /usr/share/man, mandocdb -Q now takes 45% of the time of makewhatis(8).