Rescheduling behavior on syscalls

[Subv]

Our current scheduling behavior when performing syscalls is to always attempt a reschedule, which is wrong and probably breaks some things.
The correct (although perhaps incomplete) rescheduling behavior is documented here.

This issue is a running TODO list of what's implemented and what is not.

Rescheduling

Cases when a reschedule attempt happens:

• After a call to svcExitThread.

• After a call to svcSleepThread.

• After a KThread is awakened by a timeout interrupt (WaitSynch timeout, Sleep timeout).

• In a WaitSynchronization1 call if the given timeout is greater than 0 and the thread has to wait for the object.

• In a WaitSynchronizationN call if the given timeout is greater than 0 and the thread has to wait for the objects.

• After a SyncRequest to a KClientSession.

• After a thread resumes from wait due to a KObject signal.

• After svcCreateThread.

• After svcSetThreadPriority.

• After svcReleaseMutex if the mutex lock count reached 0.

• After a KThread is destroyed and at least one thread was waiting for it.

• At the end of each SVC if the thread is marked for termination (via calling svcTerminateProcess on its parent process)

• At the end of each SVC if the thread's owner process has a KDebug event associated with it.

• After a call to svcExitProcess.

• After a call to svcTerminateProcess.

• After a KProcess is destroyed and at least one thread was waiting for it.

• After a call to svcSetResourceLimit values with resource type = CPU_TIME (9) and resource value < 0x5A.

• After a call to svcReplyAndReceive to reply to a KServerSession.

• After a KClientSession is closed. This also causes any waiting threads to awake with an error code.

• After a KServerSession is closed. This also causes any waiting threads to awake with an error code.

From here on, everything should be re-verified.

• TODO: Some magic related to AddressArbiters.
• TODO: Some magic related to memory mappings (svcControlMemory, some cache cleaning ops also cause a reschedule?).
• TODO: After a DMA transfer?
• TODO: After svcMapProcessMemory?
• TODO: After svcUnmapProcessMemory?