Don't return EPIPE from pipe kqfilter

Don't return EPIPE from pipe kqfilter

Proceed with the registration of an EVFILT_WRITE filter even if the
pipe's other end has been closed, instead of failing with error EPIPE.
The filter will trigger immediately. This reduces the possible outcomes
when a kevent(2) call and a close(2) call race on the different ends
of a pipe.

This fixes a failure of lang/ghc unit test as reported by gnezdo@.

OK gnezdo@ mpi@