The new rbtree-based scheduler makes heavy use of tv_cmp2(), and
this function becomes a huge CPU eater. Refine it a little bit in
order to slightly reduce CPU usage.
Some of the tv_* functions are called very often. Passing their
arguments as registers is quite faster. This can be disabled
by setting CONFIG_HAP_DISABLE_REGPARM.
As suggested by Markus Elfring, a few "const char *" have replaced
some "char *" declarations where a function is not expected to
modify a value. It does not change the code but it helps detecting
coding errors.
