sys: declare bit sets unsigned

Substantially reduce the number of signed/unsigned issues (warnings if
enabled). While these are presently disabled for FreeBSD, being able to
enable another warning would be good.

Differential Revision: https://reviews.freebsd.org/D32793
Reviewed by: imp, jhibbits
Pull Request: https://github.com/freebsd/freebsd-src/pull/1126