Parse `-` and `--` as non-option arguments

The `-` and `--` arguments are common in many Unix command-line tools.
`--` often means "end of options" while `-` often means "read from
standard input". Depending on the application, they can have different
meanings.

This patch makes argpar_iter_next() specifically parse `-` and `--`
arguments as non-options.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3b3f1670863992a17a7edfabdc48c921f71cd4b6