ltmain.in: Increase revision limit to 19 digits

ltmain.in: Increase revision limit to 19 digits

Current, age, and revision values for a library seem to have been chosen
arbitrarily, which is limiting users to a 5 digit maximum. Now, users
can set revision to at most 19 digits, which is referencing Unix epoch
time in nanoseconds.

Reported: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=75880

* build-aux/ltmain.in: Update revision check to allow up to 19 digits.
* tests/versioning.at: Add more version checks to test.
* NEWS: Update.