Use systemd RuntimeDirectory for /run/freeswitch

This changes how we create the temporary directory /run/freeswitch
when starting FS with systemd.  The /run directory starts empty after
a system reboot, so we need to ensure this directory is present.

As part of systemd.exec(5), systemd provides a mechanism to
automatically manage these directories under /run and to bind their
lifetime to the lifetime of the process.

This commit moves to using the RuntimeDirectory= mechanism and removes
the obsolete ExecStartPre/mkdir code.

FS-7130

debian/freeswitch-systemd.freeswitch.service

@@ -9,12 +9,12 @@ After=syslog.target network.target local-fs.target
 Type=forking
 PIDFile=/run/freeswitch/freeswitch.pid
 PermissionsStartOnly=true
-ExecStartPre=/bin/mkdir -p /run/freeswitch
-ExecStartPre=/bin/chown freeswitch:freeswitch /run/freeswitch
 ExecStart=/usr/bin/freeswitch -ncwait -nonat
 TimeoutSec=45s
 Restart=always
 ; exec
+RuntimeDirectory=freeswitch
+RuntimeDirectoryMode=0755
 WorkingDirectory=/run/freeswitch
 User=freeswitch
 Group=freeswitch