apps: decouple signal support

[extinguish]

Summary

Depencies: https://github.com/apache/nuttx/pull/17352.

Added support for signal decoupling, primarily involving modifications to the following modules in the apps: ostest, nsh/tools, netutils, and the cu tool.

1. ostest: When CONFIG_DISABLE_SIGNALS=y, certain tests that rely on signal functionality need to be disabled.
2. nsh/tools: Functionalities dependent on the signal module have been made configurable, allowing most parts of nsh to remain usable even when CONFIG_DISABLE_SIGNALS=y.
3. netutils: Ensures basic usability of certain network functionalities when signals are disabled.
4. cu tool: Refactored portions of the cu module implementation to maintain its usability under signal-disabled conditions.

Impact

No adjustments were made to the functional logic; support was only added for when CONFIG_DISABLE_SIGNALS=y.

Testing

1. has passed the ostest