Support iburst as server config option.

Allow adding 'iburst' as server option.

```
iburst
 	 When the server is unreachable, send a burst of eight packets instead of the usual one. The packet
spacing is normally 2 s; however, the spacing between the first two packets can be changed with the
calldelay command to allow additional time for a modem or ISDN call to complete. This is designed to speed
the initial synchronization acquisition with the server command and s addresses and when ntpd(8) is started
with the -q option.
```