rename tool lockstat.py to klockstat.py

The current lockstat.py is tracing three kernel functions
  mutex_lock_enter(), mutex_unlock_enter(), mutex_lock_return()
for kernel locking statistics.

There are some other efforts trying to get user lock stats by
tracing e.g. pthread locking primitives. For example,
Sasha Goldshtein's linux-tracing-workshop
   https://github.com/goldshtn/linux-tracing-workshop
is referenced in bcc/docs/tutorial_bcc_python_developer.md.
It has a tool called lockstat.py which traces pthread_mutex_init
to collect some lock statistics for userspace locks.

In bcc, in the past, we also had an effort to gather userspace
lock statistics with the same name lockstat.py.
   https://github.com/iovisor/bcc/pull/1268

In the future, bcc could have a lockstat tool tracing userspace
locks. So let us rename the current lockstat.py to klockstat.py
to clearly express its scope.