ntp: add offset option

Add offset option to the server/pool/peer directive. It specifies a
correction which will be applied to offsets measured with the NTP
source. It's particularly useful to compensate for a known asymmetry in
network delay or timestamping errors.

srcparams.h

@@ -47,6 +47,7 @@ typedef struct {
   double max_delay;
   double max_delay_ratio;
   double max_delay_dev_ratio;
+  double offset;
 } SourceParameters;
 
 #define SRC_DEFAULT_PORT 123