hush: use FEATURE_SH_NOFORK to enable NOFORK trick

Also expands docs

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

docs/nofork_noexec.txt

@@ -44,9 +44,11 @@ NOEXEC trick is disabled for NOMMU build.
 	NOFORK
 
 NOFORK applet should work correctly if another applet simply runs
-<applet>_main(argc,argv) and then continues with its business (xargs,
-find, shells can do it). This poses much more serious limitations
-on what applet can/cannot do:
+<applet>_main(argc,argv) and then continues with its business.
+xargs, find, shells do it (grep for "spawn_and_wait" and
+"run_nofork_applet" to find more users).
+
+This poses much more serious limitations on what applet can do:
 
 * all NOEXEC limitations apply.
 * do not ever exit() or exec().
@@ -56,7 +58,7 @@ on what applet can/cannot do:
     is taken from xfunc_error_retval.
   - fflush_stdout_and_exit(n) is ok to use.
 * do not use shared global data, or save/restore shared global data
-  prior to returning. (e.g. bb_common_bufsiz1 is off-limits).
+  (e.g. bb_common_bufsiz1) prior to returning.
   - getopt32() is ok to use. You do not need to save/restore option_mask32,
     it is already done by core code.
 * if you allocate memory, you can use xmalloc() only on the very first
@@ -77,3 +79,20 @@ script loops. Applets which mess with signal handlers, termios etc
 are probably not worth the effort.
 
 Any NOFORK applet is also a NOEXEC applet.
+
+
+	Relevant CONFIG options
+
+FEATURE_PREFER_APPLETS
+  BB_EXECVP(cmd, argv) will try to exec /proc/self/exe
+    if command's name matches some applet name
+  applet tables will contain NOFORK/NOEXEC bits
+  spawn_and_wait(argv) will do NOFORK/NOEXEC tricks
+
+FEATURE_SH_STANDALONE (needs FEATURE_PREFER_APPLETS=y)
+  shells will try to exec /proc/self/exe if command's name matches
+    some applet name
+  shells will do NOEXEC trick on NOEXEC applets
+
+FEATURE_SH_NOFORK (needs FEATURE_PREFER_APPLETS=y)
+  shells will do NOFORK trick on NOFORK applets