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.

data/export/systemd/master.target.erb

@@ -0,0 +1 @@
+[Unit]

data/export/systemd/process.service.erb

@@ -0,0 +1,17 @@
+[Unit]
+StopWhenUnneeded=true
+
+[Service]
+User=<%= user %>
+WorkingDirectory=<%= engine.root %>
+Environment=PORT=<%= port %><% engine.env.each_pair do |var,env| %>
+Environment=<%= var.upcase %>=<%= env %><% end %>
+ExecStart=/bin/bash -lc '<%= process.command %>'
+Restart=always
+StandardInput=null
+StandardOutput=syslog
+StandardError=syslog
+SyslogIdentifier=%n
+
+[Install]
+WantedBy=<%= app %>-<%= name %>.target

data/export/systemd/process_master.target.erb

@@ -0,0 +1,5 @@
+[Unit]
+StopWhenUnneeded=true
+
+[Install]
+WantedBy=<%= app %>.target