Implement refcounting for flocks

This was already implemented in PVE::LXC::lock_aquire() and
lock_release(). Enabling refcounting in the general
PVE::Tools::lock_file() and lock_file_full() methods allows
us to use one code base for flocking.

Furthermore, we could get rid of various xx_no_lock methods
that were required because the old non-refcounting version
did not support nested flocks (the inner most flock would
close the file handle and thus release the flock).