Control: allow for return values.

Using the property _DWM_RESPONSE. Now the client (dwm -x) waits for the server
(dwm) to set _DWM_RESPONSE. The return value could be the empty string, in which
case it still conveys the useful information "the command execution ended". The
client acquires an exclusive lock and releases it after the response was
received in order to avoid concurrency problems regarding the effective sequence
of property notify events.