Do not let the -m option or MANPATH with leading, trailing, or double

Do not let the -m option or MANPATH with leading, trailing, or double
colon override the default manpath, let them add to the default manpath.
Only override the default manpath by the -M option, by MANPATH without
leading, trailing, or double colon, or by "manpath" in man.conf(5).

Problem reported by Jan Stary <hans at stare dot cz>.
Patch OK'ed by millert@.