Enhancement and harmonization of dates in the apps

[richsalz]

From some old email...

Objects that need dates (x509 etc) will have a standard –start flag.
It takes an ISO date-time, the time defaulting to 00:00. A time and
duration can be specified by putting /duration after the start date.
Or the abosolute time can be specified with an –end flag. For example

-start 2017-02-10/p30d
-start 2017-02-10 –end 2017-03-12

Both mean the same thing, from Feb 10 for 30 days.

It is important not to break the existing flags.

Another option to consider is "-asof" to allow future and past signature checking, for example.

Also, times in the app should not use time_t but instead the ASN1 time type.

[tmshort]

Note: PR #205