Use OpenSSL built-in functions for hostname validation

We traditionally use a hand-written hostname validation algorithm in libprocess that is based on the example code in https://wiki.openssl.org/index.php/Hostname_validation

However, since OpenSSL 1.1.0, there is a new built-in function API `SSL_set1_host()` that can be used to let OpenSSL handle hostname validation during the TLS handshake in a standardized manner.

We should take advantage of this when possible.