libcupsfilters: Code clean-up for code of libcupsfilters

libcupsfilters: Code clean-up for code of libcupsfilters

Completed the code clean-up of the source files of libcupsfilters in
the cupsfilters/ directory itself (no sub-directory) 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.

This improves the readability of the code a lot, especially as missing
spaces got inserted in comma-separated lists ("xxx,yyy,zzz" -> "xxx,
yyy, zzz") and around operators ("x=a*(b+c)%4" -> "x = a * (b + c) %
4"), what got nearly completely missed out by several contributors.

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.