ape: fix thread race with close() and select()

ape: fix thread race with close() and select()

in ape close(), do the real filedescriptor _CLOSE() *after* we cleared
the _fdinfo[] slot because once closed, we dont own the slot anymore and
another process doing open() can trash the slot. make sure open() retuns
fd < OPEN_MAX.

double check in _startbuf() holding mux->lock if the fd is already buffered
preveting running double copyprocs on a fd.

dont zero the mux->rwant/ewant bitmaps at the end of select() as we do not
hold the mix->lock.

in _closebuf() kill copyproc while holding the mux->lock to make sure the
copyproc isnt holding it at the time it is killed. run kill() multiple times
to make sure the proc is gone.