LP#1635737 Add optional context to interval_to_seconds

LP#1635737 Add optional context to interval_to_seconds

Any given interval (e.g. "1 month") can be a different amount of
seconds depending on the context (i.e. "1 month" after February 1 is
March 1, but "1 month" after March 1 is April 1, yet March is longer
than February).  This affects months all the time, but also can
affect days, hours, and even seconds once you consider DST and "leap"
times.

By giving an optional context to interval_to_seconds, you can find
the true number of seconds in, for example, "1 month", when starting
from "February 1" (the context).

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Bill Erickson <berickxx@gmail.com>