Delay kserver cleanup until all tasks finishes

Delay kserver cleanup until all tasks finishes

It might be possible some pending task would run when kserver is already
cleaned up. Postpone gsstsig structures cleanup after task and timer
managers are destroyed. No pending threads are possible after it.

Make action in maybeshutdown only if doshutdown was not already called.
Might be called from getinput event.

(cherry picked from commit 2685e69be8dd6c9f81f3aebfffaaf95ba91e8934)