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.c

@@ -1436,3 +1436,13 @@ bool version_is_valid(const char *s) {
 
         return true;
 }
+
+bool version_is_valid_versionspec(const char *s) {
+        if (!filename_part_is_valid(s))
+                return false;
+
+        if (!in_charset(s, ALPHANUMERICAL "-.~^"))
+                return false;
+
+        return true;
+}