autodetect unicode, and fallback to ACS scancodes on failure

this is fairly ugly - we use a hidden ncurses function to detect
unicode (but at least we don't reimplement it), then we mess around
with the character arrays to display the rights symbols

i am not sure i like this, but i prefer to braindump than to stall, so
let's move forward.
move unicode detection to a separate function

this will ease transition to a commandline option override
add commandline flag for forcing or disabling unicode

note that "forcing" may not work: ncurses still expects your locale to
be correct, so maybe the force flag should also explicitely set a utf8
locale?
src/oping.h: Don't use the ncurses internal _nc_unicode_locale() function.

The "-u" and "-U" options are now handled in a single variable so that the
last option "wins".
Add documentation for the "-u" and "-U" options.