Change the sematics of callirq and the existing interruptors: The interrupts

Change the sematics of callirq and the existing interruptors: The interrupts
must now return carry set if the interrupt has been handled, and carry clear
if not. The callirq routine will stop calling handlers with the first handler
that claims to have handled the interrupt. callirq will return the carry flag
to the caller as it came from the last interruptor called.

git-svn-id: svn://svn.cc65.org/cc65/trunk@3491 b7a2c559-68d2-44c3-8de9-860c34a00d81