Syscall update

[Matthew Dillon]

    The next step for me is to do a bunch of cleanup on the syscall
    code.  For example, all syscalls set the return value by setting
    p_retval[0..1].  By changing all of that code to set the return
    value in the message structure I will be able to get rid of a
    large number (several dozen) struct proc references.

    I've also done a few simple performance cleanups to improve syscall
    overheads.   Don't read too much into the numbers, the biggest
    improvement will occur later on when we are able to start removing
    the MP lock.

						-Matt