Update: implement read lock handlings support.

Update: implement read lock handlings support.

There are some cases where I am able to figure out a way to make the logic continue on on failure and handle the case of read lock failure.
There are other cases where I am not yet sure how to handle.
Expect more changes in the future to address ability to continue onward on lock failure.

Some functions now return F_lock (with error bit) to designate that the lock failed.
In these cases, it is for functions that require the caller to have a read lock (such as process->lock) locked before starting.
When such functions return the caller now has a way of knowing that the read lock is no longer held.

Remove now pointless "busy" variable.

Update documentation comments with @todo.