libmn: rename yield_release() to switch_to_next_proclet() and simplify switch process

There's no need to pass a resource acquisition context through system_context()
and yield_release(). yield_release() can only be called in
system_context() so we can be confident that the resources will be acquired
and will be released when it is called, so just make sure those invariants are
held in yield_release(). Also rename to switch_to_next_proclet() since that
better reflects what it does.
libmn: remove unused includes