MFP4: (part of) 110058

MFP4: (part of) 110058

copyin()/copyout() for message type is separated from msgsnd()/msgrcv() and
it is done from its wrapper functions to support 32-bit emulations.  After I
implemented this, I have briefly referenced NetBSD and Darwin.  NetBSD passes
copyin()/copyout() function pointers from wrappers.  Darwin passes size of
message type as an argument, which is actually similar to my first
implementation (P4 109706).  We may revisit these implementations later.