Cleanup -d screenshot filename generation

[isf63]

  -d, --dirscreenshot <DIR>  Take a screenshot and save it to the directory
                             without showing the GUI (deprecated, please use
                             screengrab instead)

Previously the filenames were incomprehensible without knowledge of the code.

This also simplifies things.

[tsujan]

Previously the filenames were incomprehensible

They weren't supposed to. The goal was explained in the code clearly.

I don't see a reason for a change.

[isf63]

If anything this PR is about simplification - the filename is sane, and the code is cleaner/simpler

[tsujan]

… is about simplification

Too subjective. We have a clean static function that does the job for us and includes the pid too.