Replace all usage kill() in the OS proper with nxsig_kill().
sched/signal: Add nxsig_kill() which is functionally equivalent to kill() except that it does not modify the errno variable.
PCA9555 Driver: Replace the signalling logic with a simple callback using the new definitons of ioexpander.h. This repartitioning of functionality is necessary because (1) the I/O expander driver is the lower-lower part of any driver that uses GPIOs (include the GPIO driver itself) and should not be interacting directly with the much higher level application layer. And (2) in order to be compatible with the GPIO driver (and any arbitrary upper half driver), the PCA9555 should not directly signal, but should call back into the upper half. The upper half driver that interacts directly with the application is the appropriate place to be generating signal.
