(disassoc): Rewrite for POSIX, return status. Caller changed.

(disassoc): Rewrite for POSIX, return status.  Caller changed.
Problems with the old code:
* Insufficient error checking.
* It used TIOCNOTTY (obsolete BSDism) to get rid of the controlling
  tty, except for hpux || Rel4, where it attempted to use the POSIX
  way, but screwed up.
* It left file descriptors 0, 1, 2 in a somewhat weird state.