Support configuring multiple ECDH curves

Support configuring multiple ECDH curves

The ssl_ecdh_curve GUC only accepts a single value, but the TLS
handshake can list multiple curves in the groups extension (the
extension has been renamed to contain more than elliptic curves).
This changes the GUC to accept a colon-separated list of curves.
This commit also renames the GUC to ssl_groups to match the new
nomenclature for the TLS extension.

Original patch by Erica Zhang with additional hacking by me.

Author: Erica Zhang <[email protected]>
Author: Daniel Gustafsson <[email protected]>
Reviewed-by: Jacob Champion <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Reviewed-by: Jelte Fennema-Nio <[email protected]>
Discussion: https://postgr.es/m/[email protected]