add configure option for mail program

fixes #24

since we use @MAIL_PROG@ in our sample config, we need to provide a way to set it
by configure and at least set it to a good default.
fixed replacing mail_prog

%{bindir} is not replaced during make, use ${bindir} instead.

Signed-off-by: Sven Nierlein <Sven.Nierlein@consol.de>
Add native systemd support
Allow exclude (!) to work for regexp matches as well

Fixes #13
config: Add test for exclude with regexp matches

This supplements the parent commit. It doesn't cover all the cases, but
it looks like the (actually very simple) transform that the parent
commit does over and over does what it's supposed to.

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>