Add PERL5LIB to `opam config env`

Some opam packages (e.g. CIL) need to install perl helper scripts.
To follow perl conventions while respecting the user's expectation
that opam does not install anything outside of `opam config root`,
these scripts are installed in `opam config var lib`/perl5.

This patch modifies `opam config env` to add this directory to the
user's PERL5LIB environment variable, which allows him to use the
installed scripts seamlessly. Note that it's not an issue to set
this variable even if the directory does not exist (e.g. if no
package using perl has been installed), since perl will simply ignore
the non-existing directory.

This feature might seem out of the scope of opam, but here is an example of confused user about the CIL package:
https://bitbucket.org/zanderso/cil-template/issue/2/failing-to-build-on-mac-os-x#comment-5258619