NOFORK fixes

"rm -i FILE" and "yes" can now be interrupted by ^C in hush.
This also now works:

$ usleep 19999999
^C
$ echo $?
130

function                                             old     new   delta
run_pipe                                            1668    1711     +43
pseudo_exec_argv                                     312     321      +9
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 52/0)               Total: 52 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

docs/nofork_noexec.txt

@@ -52,6 +52,9 @@ xargs, find, shells do it (grep for "spawn_and_wait" and
 This poses much more serious limitations on what applet can do:
 
 * all NOEXEC limitations apply.
+* do not run for a long time or wait for user input:
+  hush shell only handles signals (like ^C) after you return
+  from APPLET_main().
 * do not ever exit() or exec().
   - xfuncs are okay. They are using special trick to return
     to the caller applet instead of dying when they detect "x" condition.