nls: allow composing all breved letters also with a lowercase "u"

The letters ă and ŭ can already be composed with "u a" and "u u", but
ĕ, ğ, ĭ, and ŏ can be composed only with an uppercase U.  Emancipate
the latter four and understand also a lowercase "u" to mean 'breve'.

(Yesterday I needed ğ and was annoyed that "u g" did not work.)

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>