Fix write past end of buffer

My commit f0bd5c9 changed too many things from int to size_t,
causing a write just after the end of the buffer allocated by the
earlier malloc on 64-bit systems.
tests: stop using telnet for good

ahem.
runtests: Don't start if linuxcnc or hal is already running
tests/linuxcncrsh: skip this test until we know what's up
Revert "runtests: Don't start if linuxcnc or hal is already running"

This reverts commit 0cc034c6e81873b627204904adf1c681ac8370cb.

The lockfile is currently inadequate, because it provides no way to
check if it's stale or not.  If linuxcnc crashes or otherwise doesn't
get to shut down cleanly, then the lock file will remain but be stale.