[scudo] Fix all deadlocks with condition code variables. (#212604)

Added a ScopedFLLock that automatically handles notify calls if there
are waiters. Replace all ScopedLock of FLLock with this new
ScopedFLLock. Updated the enable function to properly notify FLLock
waiters if necessary.

Added a new function for re-enabling in the child process after a fork
to clear the variables used by the condition code.

GitOrigin-RevId: 738cbf1883083525a76a6eeb30671ea778661bd3