curl: Made --metalink option toggle Metalink functionality

In this change, --metalink option no longer takes argument.  If
it is specified, given URIs are processed as Metalink XML file.
If given URIs are remote (e.g., http URI), curl downloads it
first. Regardless URI is local file (e.g., file URI scheme) or
remote, Metalink XML file is not written to local file system and
the received data is fed into Metalink XML parser directly.  This
means with --metalink option, filename related options like -O
and -o are ignored.

Usage examples:

$ curl --metalink http://example.org/foo.metalink

This will download foo.metalink and parse it and then download
the URI described there.

$ curl --metalink file://foo.metalink

This will parse local file foo.metalink and then download the URI
described there.

src/tool_help.c

@@ -125,7 +125,7 @@ static const char *const helptext[] = {
   "     --max-filesize BYTES  Maximum file size to download (H/F)",
   "     --max-redirs NUM  Maximum number of redirects allowed (H)",
   " -m, --max-time SECONDS  Maximum time allowed for the transfer",
-  "     --metalink      Process local Metalink file and use mirrors",
+  "     --metalink      Process given URLs as metalink XML file",
   "     --negotiate     Use HTTP Negotiate Authentication (H)",
   " -n, --netrc         Must read .netrc for user name and password",
   "     --netrc-optional Use either .netrc or URL; overrides -n",