core/sched: clean up

- remove stray spaces
- remove TODOs:
    - MODULE_HWTIMER is not a module anymore
    - checking for NULL is necessary, at least without API changes:
      `sched_task_exit` sets `sched_active_thread` to `NULL`, then exits,
      afterwards `cpu_switch_context_exit` calls `sched_run`
core/sched: fix pid/tcb & refactor SCHEDSTATISTICS

pid and tcb_t were compared instead of pid and pid

SCHEDSTATISTICS:
- reduce hwtimer_now calls
- dont use thread_last_pid anymore
- increase readability
core/sched: remove MODULE_NSS helpers (API CHANGE)

remove thread_getlastpid and last_pid from the API, it is unused
Merge pull request #1376 from LudwigOrtmann/sched_fixup

core/sched fixup and optimize
sys/shell: fix error when not using sixlowpan

```
/sys/shell/commands/sc_net_if.c:631:27: error: ‘IPV6_MAX_ADDR_STR_LEN’ undeclared (first use in this function)
             char addr_str[IPV6_MAX_ADDR_STR_LEN];
                           ^
```
make: model net_if dependency on net_help