bug#30285: dired-do-chmod vs. top line of dired

[Drew Adams <drew.adams@oracle.com>]

Sorry; I realize that my last msg was probably not clear
about the possibility of using `dired-move-to-filename'
or `dired-get-filename'.

Obviously, for `dired-do-*' commands we still need to
handle the case where files or dirs are marked.  And
in that case the command should DTRT even if called from
a non-file line, i.e., one of the problematic places
we've been discussing - it should not raise an error
in that context.

For `dired-do-*' commands that already call
`dired-get-marked-files' the clear solution, I think,
is to just test that return value and raise an error
if it is nil.

If there is are `dired-do-*' commands that do not call
`dired-get-marked-files' then we have a choice how to
solve the problem.  But it might well be that even then
the best solution is to use `dired-get-marked-files'.