Change TTL's y and m from fixed-length to calendar about zfsnap HOT 1 CLOSED

From the commit message

As a summary, this method is:

• Faster (doesn't spawn subshell and the external date command
• More accurate (y and m are now /calendar/ rather than 365d and 30d, respectively). Leap years are supported.
• More portable. This is POSIX compatible and doesn't rely on GNU or BSD specific features.
• More future proof. This code will handle dates past 2038 (epoch death) and prior to 1970 (not that the latter is needed).
• Fixes at least one known, unreported bug (snapshot name of 2022-09-04_02.06.00--10y is silently skipped). This is likely due to an integer overflow.

Downsides are:

• We're maintaining our own understanding of dates now, which can get very ugly. However, the previous solution already had a very rigid definition of time. This solution is more correct -- albeit not 100% correct.
• Leap seconds are still not handled (I choose not to care; only 3 in last 10 years).
• The Gregorian Calendar is still required (likely to remain a hard dependency).
• We still don't handle time zones or DST. This should be fixed in the future, but will involve real effort to not break support for pre-TZ snapshots.

from zfsnap.