the functionality of nullfs. The latter is now just a mount & unmount
routine, and a few tables. umapfs borrow most of this infrastructure.
Both fs's are now nfs-exportable.
All layered fs's share a common format to private mount & private
vnode structs (which a particular fs can extend).
Also add genfs_noerr_rele(), a vnode op which will vrele/vput
operand vnodes appropriately.
as with user-land programs, include files are installed by each directory
in the tree that has includes to install. (This allows more flexibility
as to what gets installed, makes 'partial installs' easier, and gives us
more options as to which machines' includes get installed at any given
time.) The old SYS_INCLUDES={symlinks,copies} behaviours are _both_
still supported, though at least one bug in the 'symlinks' case is
fixed by this change. Include files can't be build before installation,
so directories that have includes as targets (e.g. dev/pci) have to move
those targets into a different Makefile.
wrstuden