string-util: move version_is_valid() into generic code

While we are at it, replace the sloppy use of filename_is_valid() by the
less sloppy filename_part_is_valid() (as added by the preceeding
commit), since we don#t want to be too restrictive here. (After all,
version strings invalid as standalone filenames might be valid as part
of filenames, and hence we should allow them).

src/basic/string-util.h

@@ -275,3 +275,5 @@ char *startswith_strv(const char *string, char **strv);
 
 #define STARTSWITH_SET(p, ...)                                  \
         startswith_strv(p, STRV_MAKE(__VA_ARGS__))
+
+bool version_is_valid(const char *s);