Add support for elliptic curve keys

[jkroepke]

Pull Request (PR) description

Added support for elliptic curve keys. (easyrsa algo 'ec' and 'ed')

# The default crypto mode is rsa; ec can enable elliptic curve support.
# Note that not all software supports ECC, so use care when enabling it.
# Choices for crypto alg are: (each in lower-case)
#  * rsa
#  * ec
#  * ed

including certificate digest and ecdh_curve, since dh keys are not used if ec keys are in use.

Mention that ed25519 keys are elliptic keys, too.

This Pull Request (PR) fixes the following issues