_g_, _p_, and _m_ are primarily vestiges of the C version of the
runtime, while today we prefer Go-style variable names (generally gp,
pp, and mp).
This change replaces all remaining uses of _p_ with pp. These are all
trivial replacements (i.e., no conflicts). That said, there are several
functions that refer to two different Ps at once. There the naming
convention is generally that pp refers to the local P, and p2 refers to
the other P we are accessing.