Make msmtp as MTA separate from msmtp package proper to
allow msmtp-scripts (to be added) to use msmtp as the
backend while providing the ability to act as the
system MTA without msmtp getting in the way.

Also get rid of symlinking in postinst/postrm as that
is something the trunk devs don't like.  In addition
it allows the -mta package to conflict with any
other package providing sendmail command, as it should
(once the other packages also move postinst/postrm
sendmail symlinks to install section).

Signed-off-by: Daniel Dickinson <lede@cshore.thecshore.com>