mail/elm: Install manpages into ${PKGMANDIR}.

This project uses a Perl-style Configure script that can use a
custom config.sh to provide default answers to questions that it
asks as part of the configuration process.  Modify the custom
${FILESDIR}/config.sh to allow substituting for @PKGMANDIR@ in the
default location for unformatted manpages.

(jlam)