OpenSSL backend should respect system crypto policy
Currently we GTlsClientConnectionOpenssl and GTlsServerConnectionOpenssl set the following cipher list with SSL_CTX_set_cipher_list() by default:
#define DEFAULT_CIPHER_LIST "HIGH:!DSS:!aNULL@STRENGTH"
In Fedora and RHEL, the only allowable default cipher list is "SYSTEM", which is incompatible with all other distros because it's a downstream-specific policy. So thinking ahead to a possible future in which Fedora might build this backend, it'd be better for us to avoid calling SSL_CTX_set_cipher_list() by default (the only way to get a default policy acceptable on Fedora without adding a Fedora-specific patch), so we would only call it when G_TLS_OPENSSL_CIPHER_LIST is specified. Is there any particular rationale behind your use of "HIGH:!DSS:!aNULL@STRENGTH"?
For comparison, I just landed some work in 7ab15786 so we could drop our downstream patch changing the policy for GnuTLS.