Fix: nestable pthread cancelstate

The pthread cancelstate disable performed to ensure threads are not
cancelled while holding mutexes which are used in library destructors
does not currently support that those mutexes may be nested. It
generates error messages when using the fork and fd helpers when running
with LTTNG_UST_DEBUG=1. The effect of this is that the pthread
cancelstate can be re-enabled too soon when the first unlock is
performed (in a nested lock scenario), thus allowing the thread to be
cancelled while still holding a lock, and causing a deadlock on
application exit.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ife8b1fee04c7d7c480e59bdfc158abdee771994c