Approved by: jkh

These changes add the ability to specify that a UFS file/directory
cannot be unlinked. This is basically a scaled back version
of the IMMUTABLE flag. The reason is to allow an administrator
to create a directory hierarchy that a group of users
can arbitrarily add/delete files from, but that the hierarchy
itself is safe from removal by them.
If the NOUNLINK definition is set to 0
then this results in no change to what happens normally.
(and results in identical binary (in the kernel)).
It can be proven that if this bit is never set by the admin,
no new behaviour is introduced..
Several "good idea" comments from reviewers plus one grumble
about creeping featurism.

This code is in production in 2.2 based systems

  - add undelete() and undelete.2 (requires libc minor bump some time)
  - man page updates

say is means that a pathname had the high-order bit
set, since this is no longer an error.

in a bunch of man pages.

Use the correct .Bx  (BSD UNIX) or .At (AT&T UNIX) macros
instead of explicitly specifying the version in the text
in a bunch of man pages.