Changes

Add J9HookInterface.J9HookUnreserve() to clear flag J9HOOK_FLAG_RESERVED

J9HookRegister() sets flags J9HOOK_FLAG_HOOKED | J9HOOK_FLAG_RESERVED,
J9HookUnregister() only clears J9HOOK_FLAG_HOOKED, add J9HookUnreserve()
to clear J9HOOK_FLAG_RESERVED.
This supports an event to be registered, unregistered/unreserved, and
disabled. Otherwise, an unregistered event can't be disabled by
J9HookDisable() because of the presence of J9HOOK_FLAG_RESERVED.

Signed-off-by: Jason Feng <fengj@ca.ibm.com>
 (commit: 8159d04)