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

importers/rivendell_filter.cpp

@@ -274,7 +274,7 @@ MainObject::MainObject(QObject *parent)
                            TITLE=\"%s\",\
                            ARTIST=\"%s\",\
                            ALBUM=\"%s\",\
-                           YEAR=\"%s\",\
+                           YEAR=%s,\
                            ISRC=\"%s\",\
                            LABEL=\"%s\",\
                            CLIENT=\"%s\",\
@@ -347,7 +347,7 @@ MainObject::MainObject(QObject *parent)
 	start_datetime="null";
       }
       else {
-	start_datetime=QString().sprintf("\"%s\"",
+	start_datetime=QString().sprintf("%s",
 			(const char *)RDCheckDateTime(q1->value(7).
 			toDateTime(),"yyyy-MM-dd hh:mm:ss"));//Could be invalid (0000-00-00)
       }
@@ -355,7 +355,7 @@ MainObject::MainObject(QObject *parent)
 	end_datetime="null";
       }
       else {
-	end_datetime=QString().sprintf("\"%s\"",
+	end_datetime=QString().sprintf("%s",
 			(const char *)RDCheckDateTime(q1->value(8).
 			toDateTime(),"yyyy-MM-dd hh:mm:ss"));//Could be invalid (0000-00-00)
       }
@@ -363,7 +363,7 @@ MainObject::MainObject(QObject *parent)
 	start_daypart="null";
       }
       else {
-	start_daypart=QString().sprintf("\"%s\"",
+	start_daypart=QString().sprintf("%s",
 					(const char *)RDCheckDateTime(q1->value(16).
 					toTime(),"hh:mm:ss"));//Invalid possible?
       }
@@ -371,7 +371,7 @@ MainObject::MainObject(QObject *parent)
 	end_daypart="null";
       }
       else {
-	end_daypart=QString().sprintf("\"%s\"",
+	end_daypart=QString().sprintf("%s",
 					(const char *)RDCheckDateTime(q1->value(17).
 					toTime(),"hh:mm:ss"));//Invalid possible?
       }