exchild
return exchild(t, flags & ~XTIME, -1); /* run in sub-process */
exchild(t->left, flags|XPIPEO|XCCLOSE, pv[0]);
i = exchild(t, flags|XPCLOSE, 0);
exchild(t->left, flags|XBGND|XFORK|XCOPROC|XCCLOSE,
rv = exchild(&texec, flags, -1);
int exchild ARGS((struct op *, int, int));