ntp: rename exp1 extension field

Rename the exp1 extension field to exp_mono_root (monotonic timestamp +
root delay/dispersion) to better distinguish it from future experimental
extension fields.

candm.h

@@ -277,7 +277,7 @@ typedef struct {
 #define REQ_ADDSRC_BURST 0x100
 #define REQ_ADDSRC_NTS 0x200
 #define REQ_ADDSRC_COPY 0x400
-#define REQ_ADDSRC_EF_EXP1 0x800
+#define REQ_ADDSRC_EF_EXP_MONO_ROOT 0x800
 
 typedef struct {
   uint32_t type;