Use our own ~s ly:format placeholder, since guile is broken with wide chars

Use our own ~s ly:format placeholder, since guile is broken with wide chars

We need our own ~S format placeholder to escape quotes, so use
lilypond's string formating to make wide utf-8 chars work in filenames

Guile seems unable to handle wide characters, so we mustn't use
simple-format or format, because that will break with non-ANSI characters
(char code > 255). Instead, Implement our own ~S placeholder, that
wraps the string in double quotes and escapes backslashes, double
quotes, percent and dollar signs.

Use this format placeholder to generate the proper cmd line args
for our calls to gs.