Allocate a given signal out of the current task's pool of signals.
Every task has a set of signals to communicate with other tasks.
Half of them are reserved for the system and half of them is
free for general use. Some of the reserved signals (e.g.
SIGBREAKF_CTRL_C) have a defined behaviour and may be used by user
code, however.
INPUTS
signalNum
Number of the signal to allocate or -1 if any signal
will do.
RESULT
Number of the signal or -1 if the signal couldn't be allocated.