[Libguestfs] [PATCH nbdkit] curl: Try to share as much as possible between handles in the pool

[Richard W.M. Jones]

I'm mainly posting this to the list as a back-up.  It does work, it
does _not_ improve performance in any noticable way.  However I'm
having lots of trouble getting HTTP/2 to work (with or without this
patch) and that's stopping me from testing anything properly.

Rich.