Fix a fair number of signed/unsigned mismatches. Since these should
always be positive, unsigned is the way to go. While such mismatches
are rampant in the FreeBSD kernel, try to reduce the number in the Xen
Trying to reduce the number is a pretty gargantuan task right now. Getting rid of signed/unsigned issues also effects D30598, D30599, D30936, and D30997. I haven't fully narrowed it down, but I suspect CPU_FOREACH() needs the variable to be signed and unsigned at the same time (compares with mp_maxid which is unsigned, but CPU_ABSENT() assumes a signed argument).