lxclock: Replace named sempahore with flock

lxclock: Replace named sempahore with flock

The problem: if a task is killed while holding a posix semaphore,
there appears to be no way to have the semaphore be reliably
autmoatically released.  The only trick which seemed promising
is to store the pid of the lock holder in some file and have
later lock seekers check whether that task has died.

Instead of going down that route, this patch switches from a
named posix semaphore to flock.  The advantage is that when
the task is killed, its fds are closed and locks are automatically
released.

The disadvantage of flock is that we can't rely on it to exclude
threads.  Therefore c->slock must now always be wrapped inside
c->privlock.

This patch survived basic testing with the lxcapi_create patchset,
where now killing lxc-create while it was holding the lock did
not lock up future api commands.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>