Refactor maybe_switch_tcbs

Refactor maybe_switch_tcbs

Change maybe_switch_tcbs to return NULL when no switching is necessary.
Introduce maybe_switch_current_tcp as a thing wrapper around
maybe_switch_tcbs.

* strace.c (maybe_switch_current_tcp): New function.
(dispatch_event): Use it instead of maybe_switch_tcbs, move comments
and the os_release check before maybe_switch_tcbs invocation ...
(maybe_switch_tcbs): ... here.  Change return value to NULL
if no switching was performed.