Explicitly use an unsigned index for 'which' in shmsys(), msgsys(), and

Explicitly use an unsigned index for 'which' in shmsys(), msgsys(), and
semsys(), so the implications of the use of a negative index become obvious.
Fix the misdocumented 'which' UAP argument, which is actually signed.

Note: There is no security issue because sizeof() is unsigned but such
non-explicit comparisons are a very dangerous way to code and this fixes
that.