FS-8194 FS-7910 FS-7937 systemd service improvements

FS-8194 FS-7910 FS-7937 systemd service improvements

freeswitch-systemd.freeswitch.service:
* starting the daemon as root and switchig to freeswitch user
* respecting the options in /etc/default/freeswitch
* RuntimeDirectory parameter is replaced with a tmpfiles.d entry
  because /run/freeswitch has to be owned by freeswitch user
* instructions how to start it as non-root

debian/freeswitch-systemd.freeswitch.tmpfile:
* this defines the PID directory with correct permissions

debian/bootstrap.sh, debian/rules:
* proper handling of freeswitch.service
* deleted debian/freeswitch-systemd.install because it caused an error
  in dh_install because it's run before dh_installinit
* renamed: freeswitch-sysvinit.freeswitch.default -> freeswitch-systemd.freeswitch.default
  because sysvinit support will eventually die out

debian/freeswitch.postinst:
* run "systemctl enable freeswitch" if systemctl is available

CAVEAT: only one option is supported in /etc/default/freeswitch because the
variable ${DAEMON_OPTS} is expanded as a single token. This will be fixed
as soon as freeswitch-sysvinit is removed from freeswitch-all.