```
bsd.port.mk: Remove confusing double "/" from _DISTDIR
_DISTDIR is a variable which stores the distfiles directory of the port that is
currently being worked on. In practice, it is usually just the DISTDIR,
although some ports also define a subdirectory. Currently, a slash separating
DISTDIR from DIST_SUBDIR is added to _DISTDIR no matter if DIST_SUBDIR is set
or not. This may cause confusion to ports users by looking like a bug while it
is not.
In particular, this patch casuses diagnostic logs of failed archive extraction
to look like this:
===> Failed to extract "/portdistfiles/ttf-iosevka-ss05-4.1.1.zip".
instead of this:
===> Failed to extract "/portdistfiles//ttf-iosevka-ss05-4.1.1.zip".
```