Add usleep to newlib, silence some warnings
Fix bg=16 in terminal being interpreted as transparent

This fixes a bug in Vim and one introduced an hour ago in Nyancat.
This bug was also present in early versions of bim.

Problem: If the background is set to 16 (all black) explicitly, we still
treat it as if it is the default color and thus make it transparent.
This is wrong, only the default background, as set by resetting either
the bg itself (with mode 49) or the full display parameter set (with 0)
should show up transparent.

Solution: By using flag to indicate wether the background is "default",
we can detect other uses of background color 16 without having fudge
with other colors like 0. The flag is ANSI_SPECBG, where SPEC means
"special", and thus it is set any time the background changes
explicitly, and is unset in raw cells or places where we are in default
mode.