cpucontrol: Be more strict with input validation

[markj]

Summary

Avoid truncating 32-bit values. This would have saved me a bit of time
when I was looking at a cpuid leaf on my system and typed 0x80000001f
instead of 0x8000001f.

[kib]

usr.sbin/cpucontrol/cpucontrol.c

123

Why not strtoumax() then?

126

I do not think that usage() is required there, user does know what she did, but mis-typed the input.

[markj]

Handle kib's comments.

[markj]

usr.sbin/cpucontrol/cpucontrol.c

126

Ok, I just copied the prevailing pattern.