SCHED_ENQUEUE: tighten checks on locking

Factor out the two different implementations and ensure
modifications happen with inteerrupts disabled.  This uncovered
a latent bug in our semaphore sleep code.

Also explicitly document our locking requirements for ProcReadyList.
The comment previously claimed that proc_forbid() would offer enough
protection, which is, of course, bullshit.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@1773 38d2e660-2303-0410-9eaa-f027e97ec537