On arm and arm64, where chars are unsigned by default, buildworld dies with:
--- terminal.o ---
/usr/src/contrib/libedit/terminal.c:569:41: error: comparison of integers of different signs: 'wint_t' (aka 'int') and 'wchar_t' (aka 'unsigned int') [-Werror,-Wsign-compare]
el->el_cursor.v][where & 0370] !=
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
/usr/src/contrib/libedit/terminal.c:659:28: error: comparison of integers of different signs: 'wint_t' (aka 'int') and 'wchar_t' (aka 'unsigned int') [-Werror,-Wsign-compare]
[el->el_cursor.h] == MB_FILL_CHAR)
~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
2 errors generated.
*** [terminal.o] Error code 1
Fix this by making `MB_FILL_CHAR` a `wint_t`, so no casting is needed.