string-util: replace version_is_valid()/version_is_valid_version_spec() by a common call

Let's take inspiration from string_is_safe() and take a flags field that
allows fine tuning the validation.

Then port over all current users of either function to the new logic.
Note that this *does* change behaviour in various cases:

1. Generally: we'll now always accept the full UAPI.10 alphabet,
   including the "~" and "^" characters. As far as I can see there's no
   downside to this liberalization as none of the current consumers of
   the two functions uses these characters for anything else.

2. systemd-analyze compare-version will now accept version strings with
   "_" and "+" without complaining. I see no downside here, it just
   normalizes these debugging tools, to make them accept what most our
   other tools accept.

3. "bootctl link" will not accept empty version strings anymore
   Which is a bugfix I guess.

4. vpick will now refuse "_" and "+" in version strings. It kinda
   already did, because when parsing versions from filenames it uses "_"
   and "+" as name, architecture and attempt counter separators. We now
   systematically refuse it everywhere else in vpick too. This is hence
   a clean-up.

Fixes: #28906
Replaces: #42815 #41937

src/sysupdate/sysupdate-pattern.c

@@ -268,7 +268,7 @@ int pattern_match(const char *pattern, const char *s, InstanceMetadata *ret) {
                 switch (e->type) {
 
                 case PATTERN_VERSION:
-                        if (!version_is_valid(t)) {
+                        if (!version_is_valid(t, VERSION_ALLOW_UNDERSCORE|VERSION_ALLOW_PLUS)) {
                                 log_debug("Version string is not valid, refusing: %s", t);
                                 goto nope;
                         }