Add option to trust server's certificate

New tls policy "trust" added to /connect and /account. With the policy
TLS connection is established even with invalid certificate. Note, that
trust policy forces TLS connection and it fails when server doesn't
support TLS.

Examples:
 /connect <jid> tls trust
 /account <name> set tls trust

src/config/accounts.c

@@ -291,6 +291,7 @@ accounts_get_account(const char *const name)
         gchar *tls_policy = g_key_file_get_string(accounts, name, "tls.policy", NULL);
         if (tls_policy && ((g_strcmp0(tls_policy, "force") != 0) &&
                 (g_strcmp0(tls_policy, "allow") != 0) &&
+                (g_strcmp0(tls_policy, "trust") != 0) &&
                 (g_strcmp0(tls_policy, "disable") != 0) &&
                 (g_strcmp0(tls_policy, "legacy") != 0))) {
             g_free(tls_policy);