regexp_wrapper: return None instead of False when search times out (#1379)

This allows plugins to detect whether a search timed out or did not match, which are two distinct outcomes.

src/commands.py

@@ -163,7 +163,7 @@ def regexp_wrapper(s, reobj, timeout, plugin_name, fcn_name):
         v = process(re_bool, s, reobj, timeout=timeout, pn=plugin_name, cn=fcn_name)
         return v
     except ProcessTimeoutError:
-        return False
+        return None
 
 class UrlSnarfThread(world.SupyThread):
     def __init__(self, *args, **kwargs):