Proper file name escaping in Autoconf programs and Perl modules.

Proper file name escaping in Autoconf programs and Perl modules.

This includes escaping of characters special to the shell
as well as special to Perl, e.g., leading `<' or `>'.
For example, when $file starts with `>', `open ">$file"'
wrongly tries to append to a different file.

* bin/autoconf.as: Fix quoting for autom4te options.
* lib/Autom4te/General.pm (shell_quote): New function, taken
from coreutils, written by Jim Meyering.
(mktmpdir): Use it.
* bin/autom4te.in (files_to_options, handle_m4): Use shell_quote
and open_quote.
* bin/autoreconf.in (parse_args): Likewise.
* bin/autoscan.in (main): Likewise.
* bin/autoupdate.in (main): Likewise.
* bin/autoheader.in: Likewise, fixing old insufficient escaping.
* bin/ifnames.in: Likewise, XFile usage fixes.
* tests/tools.at (autom4te and whitespace in file names): Extend
test.  Test twice, with special characters allowed on w32, and the
rest.  Test leading and trailing whitespace, for `open_quote'.
(autotools and whitespace in file names): New, analogous test.
Reported by Paul Eggert and Benoit Sigoure, additional suggestions
by Russ Allbery and Eric Blake.