Versions of libedit that support Unicode expect that the
EL_GETCFN (the function that does character I/O) will fill in a
`wchar_t` with a character, which may be multi-byte. The built-in
function that libedit provides, but does not expose with a public API,
does properly handle multi-byte sequences.
Due to the design of Asterisk's console processing loop, Asterisk
provides its own implementation which does not handle multi-byte
characters. Changing Asterisk to use libedit's built-in function would
be ideal, but would also require changing some fundamental things
about console processing which could be fairly disruptive.
Instead, we bring in libedit's `read_char` implementation and modify
it to suit our specific needs.
Resolves: #60
