Use extended file attributes for server's file timestamp

* src/wget_xattr.h: Add fgetxattr definitions (Linux, Apple, FreeBSD)
* src/wget.c (set_file_metadata): Save "user.last_modified" attribute,
 (read_xattr_metadata): Add function,
 (write_xattr_metadata): Cleanup code,
 (http_create_request): Get timestamp from xattr if available