perl refactor: use modern semantics of 'open'

perl refactor: use modern semantics of 'open'

* lib/Automake/XFile.pm: Update comments and POD documentation to
suggest a more idiomatic/modern usage.
(open): Be more robust in detecting whether the created file handle
is being opened for writing.
* lib/Automake/FileUtils.pm (update_file, contents): Call the
'Automake::XFile' and 'File::IO' constructors with two arguments
rather than one; this change obsoletes ...
(open_quote): ... this subroutine, which has thus been removed.
(@EXPORT): Drop '&open_quote'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>