convert ntpq "when" output to second when necessary

ntpq program can produce pretty value for "when" when last packet
arrived for a long time, such as Xm (for X minutes), Xh (for X hours),
Xd (for X days). This commit converts all that pretty outputs to seconds as
diamond needs values to be float numbers.
Merge pull request #609 from hvnsweeting/handle-ntp-pretty-print

convert ntpq "when" output to second when necessary