Make sure that the configuration file is always read, even when

Make sure that the configuration file is always read, even when
running with the -M option or with a MANPATH environment variable
that has neither a leading or trailing ":" nor any "::".  If -M or
MANPATH override the configuration file rather than adding to it,
just ignore any "manpath" directives while processing the configuration
file.

This fixes a bug reported by Jan Stary <hans at stare dot cz>
on misc@.