mailwrapper

hello,

i am working on a patch for mailwrapper, which i'd like to move from
FreeBSD-utilities to its own package.  however i'm a little stymied by
usr.sbin/mailwrapper/Makefile[0], which seems to do a few fairly odd
things for the benefit of src users, such as linking mailwrapper to
either dma or sendmail if mailwrapper itself isn't built.

i'd like to significantly simplify the logic here so that if mailwrapper
is enabled (${MK_MAILWRAPPER} == yes), it's always installed in the
usual place, and doesn't pretend to be dma or sendmail, and there is no
special handling depending on the value of ${MK_SENDMAIL} and
${MK_DMAGENT}.  

this might require some changes to either sendmail or dma (which of
course i'd test before submitting anything), but in principle, does this
sound like a reasonable idea?

i am concious that many/most people don't use pkgbase yet and we
shouldn't break things for them, but this seems like an ideal to time to
clean up some of this legacy stuff.

[0] https://cgit.freebsd.org/src/tree/usr.sbin/mailwrapper/Makefile