Fix: don't use strerror() from ust lock nocheck

ust_lock_nocheck is meant to be async-signal-safe for use from the
fork() override helper (and fork(2) is async-signal-safe).

Remove calls to strerror() from ust lock functions and from the
cancelstate helper because strerror is not async-signal-safe and indeed
allocates memory.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I461f3631a24e71232d987b0a984b4942903bf9ac