Enable "." to be removed from @INC in /etc/perl/sitecustomize.pl

See CVE-2016-1238. The known vulnerable modules have been fixed, but
this should eliminate also currently unknown similar vulnerabilities
if a user chooses to uncomment the line in this file.

This facility is expected to be removed after the Debian stretch
release, at which point "." will be removed at compile time
and sitecustomize.pl will no longer have any effect.

Note that this setup leaves systems with just perl-base installed
potentially vulnerable. This is currently considered an acceptable
tradeoff.

Based on work done for jessie by Niko Tyni, adapted for sid by
Dominic Hargreaves.