pgetopts: recognise "-" as non-option and end option parsing

Currently an argument of "-" is recognised as "here are options", but as there are no options, effectively ignored. This is a bug.

I want to be able to use this as a regular command argument, often standing for "use stdin instead of a file named here". Thus, pgetopts shall see this as a normal, option-parsing-ending command argument. This is a feature.

Must be documented.