Fix error code and scheduling problem for --exclusive=[user|mcs].

When any of these --exclusive modes couldn't be satisfied, Slurm was
returning an incorrect ESLURM_NODE_NOT_AVAIL, having as a consequence
scheduling problems as described in the bug. The fix makes it so the
error code is properly set to ESLURM_NODES_BUSY, fixing also the
scheduling problems and working over the correct share_node_bitmap.

Continuation of commits from bug 4932:
e2a14b8d
fc4e5ac9

Bug 5047.