rework `namei' interface to eliminate global variables

rework `namei' interface to eliminate global variables

SCCS-vsn: sys/kern/kern_proc.c 6.3
SCCS-vsn: sys/kern/kern_sig.c 6.4
SCCS-vsn: sys/kern/vfs_vnops.c 6.2
SCCS-vsn: sys/kern/vfs_syscalls.c 6.11
SCCS-vsn: sys/ufs/ffs/ffs_vnops.c 6.11
SCCS-vsn: sys/ufs/ffs/ufs_vnops.c 6.11
SCCS-vsn: sys/ufs/lfs/lfs_vnops.c 6.11
SCCS-vsn: sys/ufs/ufs/ufs_vnops.c 6.11
SCCS-vsn: sys/kern/vfs_xxx.c 6.2
SCCS-vsn: sys/vm/vm_swap.c 6.2
SCCS-vsn: sys/kern/kern_acct.c 6.2
SCCS-vsn: sys/kern/uipc_usrreq.c 6.5
SCCS-vsn: sys/kern/kern_exec.c 6.3
SCCS-vsn: sys/ufs/ffs/ffs_vfsops.c 6.5
SCCS-vsn: sys/ufs/ffs/ufs_vfsops.c 6.5
SCCS-vsn: sys/ufs/lfs/lfs_vfsops.c 6.5
SCCS-vsn: sys/ufs/ufs/ufs_vfsops.c 6.5
SCCS-vsn: sys/vax/vax/trap.c 6.3