SSL: use X509_check_host() with LibreSSL.

SSL: use X509_check_host() with LibreSSL.

Explicit checks for OPENSSL_VERSION_NUMBER replaced with checks
for X509_CHECK_FLAG_ALWAYS_CHECK_SUBJECT, thus allowing X509_check_host()
to be used with other libraries.  In particular, X509_check_host() was
introduced in LibreSSL 2.5.0.