Instead of reopening the downloaded file, fsetxattr uses the (already
open) file descriptor to attach extended attributes. This makes the
procedure more robust against errors caused by moved or deleted files.
setxattr is a glibc call to set extended attributes, so configure now
checks for it and the code is adapted to only build when the
functionality is present.
It is often convinient to track back the source of a once downloaded
file; this patch makes curl store the source URL and other metadata
alongside the retrieved file by using the extended attributes (if
supported by the file system and enabled by --xattr).
