SSL: Add support for selecting which curves should be used by an elliptic cipher

[ChangeLog][QtNetwork][QtSSL] It is now possible to choose which elliptic
curves should be used by an elliptic curve cipher.

Change-Id: If5d0d58922768b6f1375836489180e576f5a015a
Done-with: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Richard J. Moore <rich@kde.org>