service/am: Implement EnterFatalSection and LeaveFatalSection

These functions act in tandem similar to how a lock or mutex require a
balanced lock()/unlock() sequence.

EnterFatalSection simply increments a counter for how many times it has
been called, while LeaveFatalSection ensures that a previous call to
EnterFatalSection has occured. If a previous call has occurred (the
counter is not zero), then the counter gets decremented as one would
expect. If a previous call has not occurred (the counter is zero), then
an error code is returned.