sys: declare bit sets unsigned

[ehem_freebsd_m5p.com]

Summary

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.

[ehem_freebsd_m5p.com]

This needs someone knowledgeable to look and some testing. I imagine most, if not all, uses of bit sets act as if unsigned; if any do not that would be a problem.

This is specifically targeting the CPU_FOREACH(i) macro. The current definition is:

#define CPU_FOREACH(i)                                                  \
        for ((i) = 0; (i) <= mp_maxid; (i)++)                           \
                if (!CPU_ABSENT((i)))

Trick with this is mp_maxid is a u_int which means the variable should be unsigned, otherwise you would get a warning about signed/unsigned comparison. Whereas since CPU_ABSENT(i) uses a bit set, unless the variable is signed, you get a warning about signed/unsigned conversion.

As a result without this, if -Wsign-conversion and -Wsign-compare are enabled every use of CPU_FOREACH() will result in a warning. With this, if the variable is unsigned there won't be any warnings.

[ehem_freebsd_m5p.com]

Trying for more reviewers since D32793 has simply been sitting out here gathering dust. Seems pretty appropriate as unsigned is by far the more common use.

[jhb]

I suspect unsigned is also better? Maybe @jrtc27 has thoughts? This is probably a user-facing API change since it affects cpuset_t in userland and possibly some other things.

[jrtc27]

Plain unsigned would change alignment and thus potentially ABI if someone's embedding this in a bigger structure. Changing from long to unsigned long should be fine as it's a private member and the representation isn't changing (and if you get crazy with LTO across this change I believe it should still be fine given long and unsigned long can alias).