analyze: don't warn about version spec compliant versions

analyze: don't warn about version spec compliant versions

This commits adds version_is_valid_versionspec and uses it in
analyze-compare-version.c.

version_is_valid_versionspec differs from version_is_valid in that it acepts
empty strings and since valid characters in a version spec version are all
ASCII letters and digits as well as "-.~^", but ",_+" allowed by
version_is_valid are not.

Also give a more specific warning message on invalid characters.