XFORK
execute(t, XFORK|XXCOM|XPIPEO, NULL);
rv = execute(t->left, flags|XFORK, xerrok);
flags |= XFORK;
exchild(t->left, flags|XBGND|XFORK|XCOPROC|XCCLOSE,
rv = execute(t->left, (flags&~XEXEC)|XBGND|XFORK, xerrok);
if ((flags&XFORK) && !(flags&XEXEC) && t->type != TPIPE)