Fix the edge-case of MAXTRYLOCK=0

Fix the edge-case of MAXTRYLOCK=0

We didn't really have good semantics for what is meant by MAXTRYLOCK=0,
so when skipped.exp tried it, we ended up locking the variable and then
reporting a skip without ever unlocking it.

This is now cleaning up the semantics such that MAXTRYLOCK defines how
many times we should loop if the lock is busy.  Thus MAXTRYLOCK=0 means
we try only once and fail immediately.

The testcase was accidentally creating contention due to the broken
unlock behavior.  We now have to try a bit harder to create real
contention, so some lengthy delays are inserted to hoard the lock.

* runtime/probe_lock.h (stp_probe_lock): Fix the skip behavior.
* testsuite/systemtap.base/skipped.exp: Add a big udelay.