Fix handling "C-S-u" and such when using XKB

* src/xterm.c (handle_one_xevent): Don't rely on XKB for
consumed modifiers.
2 jobs for master in 39 minutes and 49 seconds (queued for 12 seconds)
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passed #48365
build-image-inotify

00:37:36

 
  Normal
passed #48366
test-src-inotify

00:02:12