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

utils/rdimport/rdimport.cpp

@@ -1864,13 +1864,13 @@ void MainObject::WriteTimestampCache(const QString &filename,
     sql=QString().sprintf("insert into DROPBOX_PATHS set \
                            DROPBOX_ID=%d,\
                            FILE_PATH=\"%s\",\
-                           FILE_DATETIME=\"%s\"",
+                           FILE_DATETIME=%s",
 			  import_persistent_dropbox_id,
 			  (const char *)RDEscapeString(filename),
 			  (const char *)RDCheckDateTime(dt,"yyyy-MM-dd hh:mm:ss"));
   }
   else {
-    sql=QString().sprintf("update DROPBOX_PATHS set FILE_DATETIME=\"%s\" \
+    sql=QString().sprintf("update DROPBOX_PATHS set FILE_DATETIME=%s \
                            where (DROPBOX_ID=%d)&&(FILE_PATH=\"%s\")",
 			  (const char *)RDCheckDateTime(dt,"yyyy-MM-dd hh:mm:ss"),
 			  import_persistent_dropbox_id,