Revert "Add non-forking daemon mode for upstart."

This reverts commit df5763999474f52dca1fc1ac100ba65e431c511b.
Create a non-forking softetherd for upstart and systemd.

Implement a daemon that expects to be invoked by a new-style init like upstart
or systemd as:

	/usr/sbin/softetherd [vpnbridge|vpnclient|vpnserver]

Alternatively, if the command line argument is empty, then use the
`SOFTETHER_MODE` environment variable instead.
Update init files for softetherd.

The check for an upstart configuration in the sysv script is reverted because
upstart and start-stop-daemon can both control the same softetherd instance.
Create /lib/systemd/system/softether.service file.

Create a softether.upstart file in the Debian overlay for systems that use
the systemd init subsystem.
Update changelog for 4.06.9437-2~trusty release