Fix nsd and unbound's error checking with SSL_CTX_set_options().

Fix nsd and unbound's error checking with SSL_CTX_set_options().

LibreSSL, BoringSSL and OpenSSL have all been redefining SSL_OP_* flags to
be 0 when support is removed.  This breaks the error checking that nsd and
unbound were doing.  It should check against the flag value itself to allow
for 0.

input deraadt@, jsing@
tweak + ok miod@, guenther@