syscall messaging update

[Matthew Dillon]

    Another stage has been committed.  In this stage proc->p_retval has been
    removed (so everyone has to recompile libkvm/ps/top/etc... *again* :-)).
    non-error syscall results are now returned in the message structure within
    the kernel.

    I think I've managed to keep the emulation code consistent... I made sure
    that linux-mozilla still worked before I committed this mess.

    Note that for linux emulation to work properly the kern.fallback_elf_brand
    sysctl must be set to 3.

						-Matt