Use size_t for len

Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>
Add 'truecolor [on|off]' as it cannot be correctly detected

https://lists.gnu.org/archive/html/bug-ncurses/2013-10/msg00007.html

Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>