core: restrict ImportCredential= globbing

Let's restrict how we apply credential globbing in ImportCredential=, so
that we have some flexibility in automatically extending the glob
expression with per-instance data eventually without getting into
conflict with the globbing parts.

In our current uses we only allow globbing at the end of the expression,
and this is a new, unreleased feature hence let's be restrictive on this
initially. We can still relax this later if we feel the need to after
all.

Fixes: #28022

src/core/dbus-execute.c

@@ -2355,8 +2355,8 @@ int bus_exec_context_set_transient_property(
                         if (r == 0)
                                 break;
 
-                        if (!filename_is_valid(s))
-                                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Credential name is invalid: %s", s);
+                        if (!credential_glob_valid(s))
+                                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Credential name or glob is invalid: %s", s);
 
                         isempty = false;