keymap-util: also "convert" 'ru' to 'ru'

keymap-util: also "convert" 'ru' to 'ru'

As discovered by Adam Williamson in
https://bugzilla.redhat.com/show_bug.cgi?id=1333998#c32, after the changes in
81fd105a5f9 we would only match compound layouts, i.e. a comma would be
required after 'ru' to match. This seems wrong, and we should match single
layouts like too. So 'ru', 'ru,us' now both match.

startswith_comma is changed to not require a comma, i.e. check that the prefix
matches until a comma or the end of the string. Note that startswith_comma is
called twice. At the first site, we check that strings are not equal
beforehand, so this change to startswith_comma has no effect. At the second
site, it does have an effect, as described above.