Fixes for invalid NULL sql on date/times

Most files are simple swaps to get rid of extra " as a NULL return will give
invalid SQL if used as follows UPDATE x SET y="NULL";

See github issue 121 for more info.

Extra changes have been made to the following files:
* lib/rdcartslot.cpp
- Added a QDateTime variable to prevent EVENT_DATETIME becoming
  "2016-06-09" "NULL" as it was checked separately for date and time
* lib/rdcut.cpp
- Reworked START_DATETIME and END_DATETIME to save the SQL ifs.  I believe this
  makes it easier to read and understand.
- The >QDate(1900,1,1) and < 8000 is probably not needed but I left it there
  just in case.
* lib/rdescape_string.cpp
- Reworked to add extra " if the date is not NULL see issue 121
* lib/rdfeed.cpp && lib/rdpodcast.cpp
- Had to add a SetRow for QDateTime as with the string conversion the existing
  SetRow would add an extra set of "
* lib/rdsound_panel.cpp && rdairplay/log_traffic.cpp
        && utils/rddgimport/rddgimport.cpp
- Added a QString to contain EVENT_DATETIME to prevent double checks of date
  and time separately (similar to lib/rdcartslot.cpp)
* rdcatchd/rdcatchd.cpp
- Reverted the changes.  The RML here is not touching the DB so is fine, plus
  with the RDCheckDateTime changes you'd have extra " unless it was NULL
* tests/sas_switch_torture.cpp && sas_torture.cpp
- Included missing rdescape_string.h

lib/rdsvc.cpp

@@ -788,7 +788,7 @@ bool RDSvc::generateLog(const QDate &date,const QString &logname,
                            SERVICE=\"%s\",DESCRIPTION=\"%s\",\
                            ORIGIN_USER=\"%s\",ORIGIN_DATETIME=now(),\
                            LINK_DATETIME=now(),MODIFIED_DATETIME=now(),\
-                           PURGE_DATE=\"%s\"",
+                           PURGE_DATE=%s",
 			  (const char *)RDEscapeString(logname),
 			  (const char *)RDEscapeString(svc_name),
 			  (const char *)RDEscapeString(RDDateDecode(descriptionTemplate(),date)),