Merge branch 'openssl-ecp'

Uses the EVP interface for ECDH with newer OpenSSL versions, which,
compared to the previous low-level use of EC_POINT_mul() supports
hardware offloading.  We used this because of the ecp_x_coordinate_only
option, which is now removed as it's been obsolete for a long time and
complicated the code.  There is still some legacy code for OpenSSL 1.0
and the old BoringSSL version we currently use for the Android app.

Closes strongswan/strongswan#186.