perl plugin: Execute END blocks in main thread only.

perl plugin: Execute END blocks in main thread only.

Each END block is executed during perl_destruct() which happens once for each
ithread. As the number of ithreads are in no way related to any Perl plugins
this is not what we want. Now, each END block is only executed when the main
threads terminates (which happens after any other Perl thread has been shut
down).

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>