Installing downgrade script so people don't need the source tree for it.

Exception added in usage test because running the script without arguments
is valid. (This avoids the need to knownfail it).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14059

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

It is not as simple as running everything executable, because for example
.so library files are marked as executable.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

We want to be sure it says *something* and returns success.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

When a script is run with the wrong arguments, it should at least say
something like this:

    Usage: samba-foo [OPTIONS]

For many samba scripts, especially without a server environment, having
no arguments is the wrong arguments.

Here we look for every executable file with '#![...]python[3]' on the
first line, and exclude certain files and directories that have excuses
to fail the test. For example, many selftest scripts are stream-oriented
and will hang forever waiting for stdin, which is not an error. Some
test modules are designed so they can be optionally run from the command
line, but this option is typically only used by the developer who is
writing them.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>