[feature] Adding a graceful way for st-util to exit

[AJ528]

• I made serious effort to avoid creating duplicate or nearly similar issue

Per the README.md, I am opening a feature ticket before submitting my pull request.

Currently, if you terminate the st-util program using SIGINT, SIGTERM, or SIGSEGV the program will terminate but returns exit code 1. My pull request changes this behavior so that sending SIGTERM (the polite way to terminate a program) returns exit code 0. The other 2 signals still return exit code 1.