curl: Enable multi-conn for read-only connections

curl: Enable multi-conn for read-only connections

Comparing before and after this commit shows approximately double the
performance.  In other tests this allowed us to download files from
web servers at line speed.

  Benchmark 1:  nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:"
    Time (mean ± σ):     943.8 ms ±  18.8 ms    [User: 316.2 ms, System: 1029.7 ms]
    Range (min … max):   923.7 ms … 989.2 ms    10 runs

  Benchmark 2:  ~/d/nbdkit/nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:"
    Time (mean ± σ):     455.0 ms ±   6.2 ms    [User: 542.2 ms, System: 1824.7 ms]
    Range (min … max):   449.1 ms … 471.6 ms    10 runs

  Summary
    ' ~/d/nbdkit/nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:" ' ran
      2.07 ± 0.05 times faster than ' nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:" '

Multi-conn is enabled only when we know the connection is read-only:

  $ ./nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run ' nbdinfo $uri ' | grep can_multi_conn
	can_multi_conn: true
  $ ./nbdkit curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run ' nbdinfo $uri ' | grep can_multi_conn
	can_multi_conn: false

See also:
https://listman.redhat.com/archives/libguestfs/2023-February/030581.html

Reviewed-by: Eric Blake <eblake@redhat.com>