[kernel] Add event syscalls

The event object was missing any syscalls. Furthermore, kobject had an
old object_signal implementation (the syscall itself no longer exists),
which was removed. User code should only be able to set signals on
events.

src/kernel/kernel.module

@@ -53,6 +53,7 @@ kernel = module("kernel",
         "syscalls.inc.cog",
         "syscalls/channel.cpp",
         "syscalls/endpoint.cpp",
+        "syscalls/event.cpp",
         "syscalls/handle.cpp",
         "syscalls/object.cpp",
         "syscalls/process.cpp",