Process command line arguments (getopt cover).

processArgs calls getopt to process command line arguments. It does any additional validation and parameter derivations needed. A tuple is returned. First value is true if command line arguments were successfully processed and execution should continue, or false if an error occurred or the user asked for help. If false, the second value is the appropriate exit code (0 or 1).

Returning true (execution continues) means args have been validated and derived values calculated. In addition, field indices have been converted to zero-based.

struct TsvSelectOptions
ref string[] cmdArgs