make heavier use of process_lock (v2)

pthread_mutex_lock() will only return an error if it was set to
PTHREAD_MUTEX_ERRORCHECK and we are recursively calling it (and
would otherwise have deadlocked).  If that's the case then log a
message for future debugging and exit.  Trying to "recover" is
nonsense at that point.

process_lock() was held over too long a time in lxcapi_start()
in the daemonize case.  (note the non-daemonized case still needs a
check to enforce that it must NOT be called while threaded).  Add
process_lock() at least across all open/close/socket() calls.

Anything done after a fork() doesn't need the locks as it is no
longer threaded - so some open/close/dups()s are not locked for
that reason.  However, some common functions are called from both
threaded and non-threaded contexts.  So after doing a fork(), do
a possibly-extraneous process_unlock() to make sure that, if we
were forked while pthread mutex was held, we aren't deadlocked by
nobody.

Tested that lp:~serge-hallyn/+junk/lxc-test still works with this
patch.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Tested-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>