Allow certificate lifetimes to be given in d,mo,y as well as hours.

[sshipway]

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Issue details

Currently, certificate lifetimes can only be given in hours, minutes and seconds. If you want to create a cert with a 90 day life, you cant say --not-after 90d but instead have to put --not-after 2160h

We should be able to use time periods in days or years; months might be awkward as they are different lengths and the 'm' would clash with 'minutes' but maybe '1M' could mean '30d'?

I would like to be able to use time windows such as

• --not-after 1y
• --not-after 90d
• --not-after 3mo (this one I could live without though)

Why is this needed?

This is a relatively simple change that would be a bit improvement on usability when creating longer-lived certificates such as for hosts and services, which typically want to live for 90 days or 1 year. Giving longer times in hours is confusing and error-prone.