boolean_t: change to unsigned int to avoid signed bitfield warnings

This is the final part, which actually makes boolean_t unsigned. Note
that we do not change its size, nor do we try to change it directly to
bool, since that results in a lot of regressions.

Converting the remaining instances of boolean_t to plain C99 bool can
now be done in a piecemeal fashion, after which boolean_t may hopefully
be retired.

MFC after: 1 week
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D39753