Now the logic for handling name duplicates and picking the longest
expiry and strictest subdomain is the same for the callback as for when
reading from file.
Also strip trailing dots from the hostname added by the callback.
A minor side-effect is that the hostname provided by the callback can
now enable subdomains by starting the name with a dot, but we discourage
using such hostnames in documentation.
Amended test 1915 to verify.
Closes#21201