avoid symlink attacks in launch_tcpcryptd.sh

/tmp is world-writable -- if a non-privileged user creates a symlink
in /tmp/tcpcrypt.pid pointing to some file, then the superuser
executing launch_tcpcryptd.sh will truncate that file.

Modern systems will use /run or /var/run for this sort of thing,
rather than /tmp, since /var/run is not world-writable.