uterm: input: fix key-repeat handling

uterm: input: fix key-repeat handling

We currently do not handle modifier-changes during key-repeats. This is
odd as pressing shift should change a repeating key.

To avoid duplicating a lot of code, this patch puts most of the
key-handling into helper functions and cleans it up. We now handle all
kinds of key-repeat specialties and everything should work fine.

Reported-by: Ran Benity <ran234@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>