Patch for cups.py to warn about dangerous LC_CTYPE

Please apply the attached patch to prnt/cups.py.

It is very simple:

     if current_ctype is not None:
+ #### Note: if does not help to set LC_CTYPE to C above (from
+ #### newctype). This setting here matters.
         os.environ['LC_CTYPE'] = current_ctype
+ if len(current_ctype) > 0 and current_ctype != "C":
+ log.warn("LC_CTYPE is " + current_ctype + ". If a PPD file cannot be found: cancel, unset your LC_CTYPE environment variable, and rerun.")

It adds a warning if LC_CTYPE is set to anything other than C. With LC_CTYPE set to en_US, at least, hp-setup is not able to find the PPD file for a Photosmart C7280. With this patch, I get a useful warning from both hp-check and hp-setup. Without it, this problem is difficult to track down. See, for example,
https://bugs.launchpad.net/hplip/+bug/225970 and
https://bugs.launchpad.net/hplip/+bug/162196
I expect this this problem has caused grief for many, many other people.

The patch also includes a comment noting that attempting to set LC_CTYPE to C prior to the "import cupsext" doesn't solve the problem.

An alternative approach to solving this problem might be to replace the warning with a hard-coded assignment of LC_CTYPE to C here. I don't know if that is a good idea or not.

Here is how I tested this patch:
# cd /usr/share/hplip/ #### Where the prnt/ directory containing cups.py* is located.
# patch < /tmp/cups.py.diff
# env -i PYTHONPATH=. python -i cups.py
# env -i PYTHONPATH=. python -i -O cups.py
$ LC_CTYPE=en_US hp-check #### Triggers the warning.
$ LC_CTYPE=en_US hp-setup #### Triggers the warning.
$ LC_CTYPE= hp-setup #### Allows successful setup!