Make Pod::Perldoc use PERLDOC_POD2 ENV var

Make Pod::Perldoc use PERLDOC_POD2 ENV var

* The "PERLDOC_POD2" environment variable could be used, to set once, the
preferred translation (instead of using "-L" switch each time).

eg. export PERLDOC_POD2=it

If "PERLDOC_POD2=1", the language code will be auto-retrieved from
locale, that is, from "LC_ALL" "LC_LANG" "LANG" environment variables
respectively.

export PERLDOC_POD2=1

* bug fix: -f switch works correctly with missing translations.
* bug fix: perldoc now correctly works with several translators