include/opt: Remember arguments passed to opt_parse()

include/opt: Remember arguments passed to opt_parse()

To allow the user to distinguish running instances of a script, it is
necessary to present the arguments that an instance was started with.
The most logical place to remember such information is inside the opt
module. Therefore, this commit adds the opt_get_argv() function, which
allows the caller to retrieve the arguments that were passed to the
opt_parse() function.