tunables: Fix comparison of tunable values

The simplification of tunable_set interfaces took care of
signed/unsigned conversions while setting values, but comparison with
bounds ended up being incorrect; comparing TUNABLE_SIZE_T values for
example will fail because SIZE_MAX is seen as -1.

Add comparison helpers that take tunable types into account and use
them to do comparison instead.

(cherry picked from commit d1a3dcab)