bitmap: use unsigned int for iterating over bitmap words

Bitmap words (e.g. resulting from BITMAP_{N,HEAD}WORDS) are of
unsigned type.

Use "unsigned int" to iterate over bitmap words to avoid comparisons
between signed and unsigned expressions. GCC otherwise warns about
these when -Wsign-compare is enabled.

Signed-off-by: Emilio G. Cota <cota@braap.org>
io: io_never for events that should never happen.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
io: change io_idle() to io_wait()

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
io: don't close if already closing in io_close_other.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>