GUACAMOLE-1956: Rename SSL client auth properties to use "ssl-auth" prefix (not just "ssl").

The "ssl" prefix sounds like it's meant for configuring general use of
SSL/TLS for encryption in front of Guacamole, which is confusing,
particularly when encountering the "SSL_ENABLED" environment variable.

The "ssl-auth" prefix and "SSL_AUTH_ENABLED" environment variable don't
suffer from the same issue.

guacamole-docker/build.d/010-map-guacamole-extensions.sh

@@ -110,7 +110,7 @@ map_extensions <<'EOF'
     guacamole-auth-sso/cas......................CAS_
     guacamole-auth-sso/openid...................OPENID_
     guacamole-auth-sso/saml.....................SAML_
-    guacamole-auth-sso/ssl......................SSL_
+    guacamole-auth-sso/ssl......................SSL_AUTH_
     guacamole-auth-totp.........................TOTP_
     guacamole-display-statistics................DISPLAY_STATISTICS_
     guacamole-history-recording-storage.........RECORDING_