lib: add ability to log from external pthread

lib: add ability to log from external pthread

External libraries can re-enter the FRR code through a hook function. A
crash occurs when logging from this hook function if the library has
initiated a new pthread, as the FRR RCU context is not initialized for
this thread.

Add frr_pthread_non_controlled_startup() function to initialize a valid
RCU context within a FRR pthread context, originating from an external
pthread.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>