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.

src/basic/string-util.h

@@ -282,3 +282,5 @@ char *startswith_strv(const char *string, char **strv);
         startswith_strv(p, STRV_MAKE(__VA_ARGS__))
 
 bool version_is_valid(const char *s);
+
+bool version_is_valid_versionspec(const char *s);