Fix poll() on half-closed sockets, while retaining POLLHUP for fifos.

This reverts part of r196460, so that sockets only return POLLHUP if both
directions are closed/error. Fifos get POLLHUP by closing the unused
direction immediately after creating the sockets.

The tools/regression/poll/*poll.c tests now pass except for two other things:

• if POLLHUP is returned, POLLIN is always returned as well instead of only when there is data left in the buffer to be read
• fifo old/new reader distinction does not work the way POSIX specs it

Reviewed by: kib, bde