pulled out the duplicated, conditional code from both kern/vfs_syscalls.c

pulled out the duplicated, conditional code from both kern/vfs_syscalls.c
and compat/common/vfs_syscalls_43.c and placed a single copy of that code
into miscfs/union/union_subr.c (seemed like a good place to put it, since
it's union-fs related).
as a side effect you can build unionfs in lkm.
(netbsd pr#2950, Paul Goyette <paul@pgoyette.bdt.com>)