Force files overwrite with USE_ZIP=yes

[Pete Fritchman]

++ 02/06/03 03:48 +0700 - Alexey Dokuchaev:
| While, when [gb]ziped distfiles explicitly set to overwrite existing
| files upon extraction, this is not the case with .zip-suffixed
| distfiles, thus requiring explicit override of EXTRACT_BEFORE_ARGS for
| some ports that need this.  I therefore propose the following change to
| /usr/ports/Mk/bsd.port.mk to make things easier for porters and general
| consistency with GZIP/BZIP2 distfiles behavior.

This looks like a good idea; could you submit a PR with this patch so
portmgr@ has it in their queue?

Thanks,

--pete