Cleanup of per-input global variables in automake.in.

Cleanup of per-input global variables in automake.in.

* automake.in ($output_vars, $output_all, $output_header)
($output_rules, $output_trailer): Fix comment, and sort
declarations in the order they will appear in the output.
($all_target): Unused variable, remove.
(Variables not reset by &initialize_per_input.): New section,
clarify that %am_file_cache is not reset.
(initialize_per_input): Reset $am_file_name, $am_relative_dir,
$in_file_name, $relative_dir, and $topsrcdir to `undef' rather
than the empty string; $topsrcdir was not reset at all.  Sort
remaining variables in the same order as in the section
`Variables reset by &initialize_per_input'.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>