FS-7928 FS-7618 systemd and package build improvements

debian/bootstrap.sh:
 * only build one of freeswitch-sysvinit or freeswitch-systemd
 * squeeze is removed from supported releases
 * added stretch to supported releases
 * avoid_mods_wheezy extended to modules which fail to build on wheezy
 * use systemd by default for future distros
 * new command-line option -v to enforce sysvinit
 * added dependency on dh-systemd for systemd-powered distros
 * freeswitch-init is now a virtual package
 * freeswitch-sysvinit and freeswitch-systemd are set to conflict with each other

debian/freeswitch.postinst:
 * no need to call systemctl explicitly. dh-systemd does it in a standard way

debian/rules:
 * integrated dh-systemd in override_dh_installinit

debian/freeswitch-systemd.freeswitch.default renamed to
freeswitch-sysvinit.freeswitch.default:
 * /etc/default/freeswitch is not installed by freeswitch-systemd, but still
   respected if there is a need to modify the startup options

debian/freeswitch-systemd.freeswitch.service:
 * proper expansion of DAEMON_OPTS

debian/control-modules

@@ -611,7 +611,6 @@ Description: VLC streaming
 Build-Depends: libvlc-dev (>= 2.0.0)
 Depends: vlc-nox
 Suggests: vlc-dbg
-Distro-Conflicts: squeeze
 
 Module: formats/mod_webm
 Description: Adds mod_webm