* Adds the send_once command and message type, which allows a user to request a variable to be sent immediately and only once (intended to replace the var_add, var_send, var_clear idiom that is commonly used for this purpose)
* Minor refactoring of variable server internals to reduce repeated code
* Adds SIM_test_varserv to integration test to test basic variable server functionality
* Changes graphics client for SIM_billiards to use var_send_once as an example of intended use
* Add documentation for var_send_once in docs and tutorial
* Set exit_code_enabled in trick unit tests to be true by default
* Patch for failing bookworm build
Novice linux/trick users don't catch that "trick" and "trick.git" are incompatible, because they don't understand that the file extension is a part of the directory name. For this reason they frequently leave the ".git" in their shell environment and they get stuck there for a while. While it is important that trick users have an understanding of how their shell uses the PATH variable, I do not think this little section at the bottom of the intro should have to be debugged so frequently.
