rough draft for systemd export support

http://0pointer.de/blog/projects/systemd.html

This adds support for exporting systemd targets and services.  The
structure is based on the existing upstart support.

Quality is draft and expected to refine in the coming weeks.

One Foremanism that is not respected by these export templates is the
usual log output location, instead stdout and stderr go to syslog.
better default for things which intentionally daemonize child processes, the default KillMode is control-group which survives daemonization
fix spec after d4ab495
updated systemd export spec after rebasing included 5ef8bbdb
Merge pull request #294 from bfulton/master

add systemd export