CUPS filters: Code clean-up for code in filter/

CUPS filters: Code clean-up for code in filter/

Cleaned up the code of the legacy CUPS filters/filter function
wrappers following the coding style rules in the DEVELOPING.md file of
the CUPS source code.

Comments are re-formatted to use "// ..." instead of "/* ... */", like
in PAPPL, so C and C++ files get the same comment style.

Also we get rid of the mix of many different coding styles which came
together from the many code contributions received during more than a
decade, even before the start of the cups-filters project.

In addition, all the file's header comments reflect the new license
now, Apache 2.0, the same license as used for CUPS.