Further prototyping and cleanup of UKERNEL

Further prototyping and cleanup of UKERNEL

This patch continues to reduce the number of warnings that come from
a UKERNEL build. It also replaces the use of a system provided suser()
call (which isn't available everywhere), with afs_suser that's provided
by UKERNEL. Note that this makes UKERNEL applications usable by
everyone, not just root.

fork_syscall() becomes a normal function
call_syscall() is ANSIfied
UKERNEL/osi_groups.c gets the includes it needs
UKERNEL/osi_machdep.c stops thinking it's on Solaris
UKERNEL/osi_vnodeops.c stops confusing vnodes and vcaches

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/166
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>