Log Message:
Update to version 0.12.  While here, add full DESTDIR support.
I've also included pthread.buildlink3.mk to define PTHREAD_CFLAGS et. al.
for configure and USE_FEATURES.openssl=threads.

Changes:

btpd 0.12:
----------

Misc:
- btpd now starts with the same set of active torrents as it had when it was
  last shut down. The new '--empty-start' option disables this feature.
- btpd now supports the key tracker extension.
- btpd now supports the multi tracker extension.
- btcli list and stat print headers for the presented data. Both commands
  displays more data than before.
- Torrent management has changed, with changes to the add and del commands
  and two new commands, start and stop, as a result. Operation on torrents
  can now be done by number as well as torrent file.
- A small delay has been added for contacting the same announce URL.
- btpd sends keepalive messages.
- The number of have messages sent has been reduced by not sending them to
  peers that already have the piece.
- The structure of the torrents directory in the btpd directory has changed.
  It is best to remove it before running btpd 0.12.

Bug fixes:
- Zero sized files in a torrent were not always created.