Issue 3832: react sanely to ill-formed version strings

Issue 3832: react sanely to ill-formed version strings


Contains commits

Clean up Lilypond_version

Converting to int seems strange, but we want version comparisons.
Conversion to bool for checking validity seems nice.

And bombing out because of invalid data to String_convert::dec2int
seemed definitely non-nice.  This version requires specifying at
least major and minor version number explicitly separated by dots.

It accepts a trailing dot, too.  A trailing extra_patch_string after
major.minor.patchlevel. is also accepted but not interpreted.

Don't verify result of String_convert::dec2int

The error behavior is not useful and triggers on things like "00".