getopthelp: Fix comments

Consistent comment-block style. Make valid sentences.

Signed-off-by: Celelibi <celelibi@gmail.com>
getopthelp: Added argidx field

This field is set to the first non-option argument index in argv. So
that the caller can reach its mandatory arguments. It is equivalent to
optind for getopt.

Signed-off-by: Celelibi <celelibi@gmail.com>
filtered_string: Module for accessing strings

It allows the caller to access only the characters that belong to a
given charset without having to modify the real string.

Signed-off-by: Celelibi <celelibi@gmail.com>