all over the place: use sched_active_pid

In many places we needlessly use `sched_active_thread->pid` whilst we
already have `sched_active_pid` with the same value, and one less
indirection.

`thread_getpid()` is made `static inline` so that there is no penalty in
using this function over accessing `sched_active_pid` directly.
Merge pull request #1575 from Kijewski/use-sched_active_pid

all over the place: use sched_active_pid