hush: use FEATURE_SH_NOFORK to enable NOFORK trick

Also expands docs

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/libbb.h b/include/libbb.h
index e69e279..88dceb1 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -859,6 +859,7 @@
 # define update_utmp(pid, new_type, tty_name, username, hostname) ((void)0)
 #endif
 
+
 int execable_file(const char *name) FAST_FUNC;
 char *find_execable(const char *filename, char **PATHp) FAST_FUNC;
 int exists_execable(const char *filename) FAST_FUNC;