libppd: Code clean-up for code of libppd

libppd: Code clean-up for code of libppd

Cleaned up the code following the coding style rules in the
DEVELOPING.md file of the CUPS source code.

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.

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,
Apache 2.0, the same license as used for CUPS.