fix masking signals when using multiple threads

Signals are blocked in main thread, but are not blocked on all other thread pool/receiver threads. The SIGCHLD must be masked before spawning the new thread and then restore the signal mask.

Class affected by this:

ThreadPool
WatchedThread
ProcessSpawnImpl which is used by LFCSetup/LFC (forked from the Kea server when using memfile) - the one registering signal handle on SIGCHLD.

Edited Jan 21, 2021 by Razvan Becheriu

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