[kernel] Replace endpoint with new mailbox API

The new mailbox kernel object API offers asynchronous message-based IPC
for sending data and handles between threads, as opposed to endpoint's
synchronous model.

src/kernel/kernel.module

@@ -32,9 +32,9 @@ kernel = module("kernel",
         "memory_bootstrap.cpp",
         "msr.cpp",
         "objects/channel.cpp",
-        "objects/endpoint.cpp",
         "objects/event.cpp",
         "objects/kobject.cpp",
+        "objects/mailbox.cpp",
         "objects/thread.cpp",
         "objects/process.cpp",
         "objects/system.cpp",
@@ -52,9 +52,9 @@ kernel = module("kernel",
         "syscall_verify.cpp.cog",
         "syscalls.inc.cog",
         "syscalls/channel.cpp",
-        "syscalls/endpoint.cpp",
         "syscalls/event.cpp",
         "syscalls/handle.cpp",
+        "syscalls/mailbox.cpp",
         "syscalls/object.cpp",
         "syscalls/process.cpp",
         "syscalls/system.cpp",