Expose arg_basename function as part of the API

[antoniovazquezblanco]

Given that arg_basename is used internally in arg_file, I was wondering if it would make sense to expose it to the user.

One simple but common use would be to substitute progname with arg_basename(argv[0]).

Thanks

[tomghuang]

@antoniovazquezblanco : I use the cwalk library (https://likle.github.io/cwalk/reference/) to do these path operations. It is a single-file library, which is as easy to use as argtable3.

[antoniovazquezblanco]

Thanks!