Generalise Banner Code

[oubiwann]

Tasks:

• [ ] Support backends that are ready immediately (e.g., bevin backend) as well as those that need to start up another service (e.g., extempore)
  • [ ] This will mean two paths for banner-rendering ... the app-level for OS process backends
  • [ ] A check for msgs from the service (e.g., in the tcp-client callback)
• [ ] Add configuration for type of banner-renderer
• [ ] Add configuration for delay (useful for banner being rendered with timer:apply_after)
• [ ] Clean up version and banner code in:
  • [ ] apps/undertone/src/undertone/bevin.lfe: extract-version
  • [ ] apps/undertone/src/undertone/sysconfig.lfe: lots of functions
  • [ ] apps/undertone/src/undertone/xtrepl.lfe
  • [ ] src/xt/msg.lfe