Fix select/poll/kqueue for write on reverse direction before first write.
Fix select/poll/kqueue for write on reverse direction before first write.

The reverse direction of a pipe is lazily allocated on the first write in
that direction (because pipes are usually used in one direction only).  A
special case is needed to ensure the pipe appears writable before the first
write because there are 0 bytes of pending data in 0 bytes of buffer space
at that point, leaving 0 bytes of data that can be written with the normal
code.

Note that the first write returns [ENOMEM] if kern.ipc.maxpipekva is
exceeded and does not block or return [EAGAIN], so selecting true for write
is correct even in that case.

PR:		kern/93685
Submitted by:	gianni
MFC after:	2 weeks

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