kernel -- syscons: Resolve unmatched PHOLD() for MOUSE_MODE ioctl.

kernel -- syscons: Resolve unmatched PHOLD() for MOUSE_MODE ioctl.

When a syscons process put itself into MOUSE_MODE, a process-hold
was being placed to stabilize the process for signal delivery. The
hold was not being released on process exit, however.

This change reworks syscons to install a per-process flag as to
whether it is in MOUSE_MODE or not and to remove the excess hold on
exit(). It also cleans up the error handling and prevents stale
process pointers from lingering in syscons stat structures.

Reported-by: mneumann, Studbolt
Closes-bug: 2521