Use systemd RuntimeDirectory for /run/freeswitch

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