Add support to save file extended attributes

• configure.ac: Check for xattr availability
• src/Makefile.am: Add wget_xattr.h
• src/options.c: Add new config 'xattr', (parse_execute): Add new option 'xattr
• src/wget.c: (_prepare_file): Add condition for 'xattr', (http_send_request): Add parameter 'original_url', (add_url): Assign 'original_url' with proper value, (download_thread): Add variable 'original_url', (write_xattr_metadata): New function, (set_file_metadata): New function
• src/wget_job.h: Add variable 'original_url'
• src/wget_options.h: Add new config 'xattr'
• src/wget_xattr.h: Add new function declaration 'set_file_metadata', Add fsetxattr() if defined in operating system

These attributes provide a lightweight method of later determining where a file was downloaded from. Adapted from Wget 1.x.

This patch changes:

• autoconf detects whether extended attributes are available and enables the code if they are.
• The new flags --xattr and --xattr=no control whether xattr is enabled.
• The original and redirected URLs are recorded as shown below.
• This works for both single fetches and recursive mode.

The attributes that are set are:
user.xdg.origin.url: The URL that the content was fetched from.
user.xdg.referrer.url: The URL that was originally requested.
user.mime_type: Two-part identifier for file formats and format contents.
user.charset: File encoding.

For example, where http://lwn.net redirects to https://lwn.net:
$ wget2 --xattr http://lwn.net
...
$ getfattr -d index.html
user.charset:"UTF-8"
user.mime_type:="text/html"
user.xdg.origin.url="https://lwn.net/"
user.xdg.referrer.url="http://lwn.net/"

TODO:

• Add test case
• Add docs

Still far from complete. Here some minus I found:

• At src/wget.c, when calling set_file_metadata, I still don't know how to pass value of URI.
• When built, it break ./test/test-wget-1.

Based on #210 (closed)