various tools: be more explicit when a glob is passed when not supported

See https://bugzilla.redhat.com/show_bug.cgi?id=1763488: when we say that
'foo@*.service' is not a valid unit name, this is not clear enough. Let's
include the name of the operation that does not support globbing in the
error message:

$ build/systemctl enable 'foo@*.service'
Glob pattern passed to enable, but globs are not supported for this.
Invalid unit name "foo@*.service" escaped as "foo@\x2a.service".
...

src/nspawn/nspawn-register.c

@@ -258,7 +258,7 @@ int allocate_scope(
         if (r < 0)
                 return log_error_errno(r, "Could not watch job: %m");
 
-        r = unit_name_mangle_with_suffix(machine_name, 0, ".scope", &scope);
+        r = unit_name_mangle_with_suffix(machine_name, "as machine name", 0, ".scope", &scope);
         if (r < 0)
                 return log_error_errno(r, "Failed to mangle scope name: %m");
 
@@ -350,7 +350,7 @@ int terminate_scope(
         _cleanup_free_ char *scope = NULL;
         int r;
 
-        r = unit_name_mangle_with_suffix(machine_name, 0, ".scope", &scope);
+        r = unit_name_mangle_with_suffix(machine_name, "to terminate", 0, ".scope", &scope);
         if (r < 0)
                 return log_error_errno(r, "Failed to mangle scope name: %m");