libcupsfilters: Allow filter without path in filterExternalCUPS()

libcupsfilters: Allow filter without path in filterExternalCUPS()

The filterExternalCUPS() function now also works if the filter (or
backend) is specified without absolute path. In this case the
CUPS_SERVERBIN environment variable (does not matter whether it was
set in the actual environment or in the parameters of
filterExternalCUPS()) is used to locate the filter or backend, using
the /filter or /backend subdirectory of the directory specified by
CUPS_SERVERBIN.

In addition, this directory is also added to the beginning of PATH, as
CUPS does, so that filters/backends can find auxiliary programs (or
other filters if they are a wrapper) in the same directory where they
themselves are located.