syscall messaging 3: Expand the 'header' that goes in front of the syscall

syscall messaging 3: Expand the 'header' that goes in front of the syscall
arguments in the kernel copy.  The header was previously just an lwkt_msg.
The header is now a 'union sysmsg'.  'union sysmsg' contains an lwkt_msg
plus space for the additional meta data required to asynchronize various
system calls.   We haven't actually asynchronized anything yet and will not
be able to until the reply port and abort processing infrastructure is
in place.  See sys/sysmsg.h for more information on the new header.

Also cleanup syscall generation somewhat and add some ibcs2 stuff I missed.