Add ECDSA support

This commit adds the possibility to specify the algorithm used by openssl
by adding the --use-algo and --curve parameters.

Signed-off-by: Steffan Karger <steffan@karger.me>
Update to ECDSA support from commit 45c27e9

Add documentation & help output; add quoting; add error messages; put
the ecparams dir under PKI dir with env-var tunable.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>