Refactor the cu tool

[extinguish]

Summary

1. Merged cu.h into cu_main.c to simplify the code structure and reduce file dependencies.
2. Implemented CU exit logic following the pattern used in the top command, using a local cu_globals_s instance to manage the CU exit process.

Impact

1. Reduces separation between header and source files, improving code maintainability and readability.
2. Introduces a more controlled CU exit mechanism, enhancing program stability and predictability during shutdown, and preventing potential resource leaks.

Testing

Verified that CU compiles and runs correctly in a local environment, with the exit process behaving as expected.