in forked process.
(interruptible): Flag as interruptible when running in main process module.
(interrupt_setup): Save return address and address of return address.
(signal_fixup_after_fork): New function. Uses above two values to restore
proper behavior to forked process.
(interrupt_on_return): Pass return address address to interupt_setup.
(interrupt_now): Pass NULL for return address address to interrupt_setup.
* fork.cc (fork): Call signal_fixup_after_fork.
* shared.h: Lint cleanups.
* winsup.h: Ditto.
