bug#11884: 24.1.50; a regression with pselect(2) on FreeBSD after r108687

[Jan Beich]

Trying to use anything that waits for a process turns emacs into a cpu
hog, e.g. browse-url or vc-annotate. A simple test case is:

  $ emacs -Q
  (start-process "dummy" "*dummy*" "/usr/bin/true")

It does nothing on r108686 but degrades on r108707 (intermediate
revisions do not compile). list-processes still shows `true' despite the
process is long gone:

  Process [v]     Status  Buffer          TTY          Command
  dummy           run     *dummy*         /dev/pts/8   /usr/bin/true

in ktrace(1) it looks like this (repeatedly)

   911 emacs    CALL  ioctl(0x3,FIONREAD,0x7fffffffb47c)
   911 emacs    RET   ioctl 0
   911 emacs    CALL  
pselect(0x5,0x7fffffffc850,0x7fffffffc7d0,0,0x7fffffffc7c0,0)
   911 emacs    RET   pselect 1
   911 emacs    CALL  ioctl(0x3,FIONREAD,0x7fffffffb46c)
   911 emacs    RET   ioctl 0
   911 emacs    CALL  read(0x4,0x7fffffffb550,0x1000)
   911 emacs    GIO   fd 4 read 0 bytes
       ""
   911 emacs    RET   read 0

--
FreeBSD 10.0-CURRENT r237800 amd64