Fixed several bugs related to parsing CLI arguments, added comments, …

[AllenEveland]

• Instead of using the classic enum {}, switch to #define.
• Replace while(1)/break with do-while(p != NULL) to make the loop's exit condition explicit and remove the dead matched_any guard.
• Fixed a bug where users passed empty values ỏ invalid path(may be other flags or something) ​​to the parameters for -c and --config, and added a feature to check for existing paths before proceeding.