Wrap the functions that require it in PRIVATE_KEY_UNLOCK/PRIVATE_KEY_LOCK.
This can't be done at plugin initialization because it needs to be done
for every thread. strongSwan currently doesn't provide on-thread-create
callbacks for plugins so we need to wrap each direct call. Another reason
to do so is that some functions we call (e.g. wc_EccKeyToDer) internally
call PRIVATE_KEY_UNLOCK/PRIVATE_KEY_LOCK and would leave the keys locked
for that particular thread.
Juliusz Sosinowicz