Mk/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".

PR:		282291
Reviewed by:	pizzamig
Approved by:	pizzamig (portmgr)
Event:		Berlin Hackathon 202507
Differential Revision:	https://reviews.freebsd.org/D27722

Mk/bsd.port.mk

@@ -1007,7 +1007,7 @@ PORTSDIR?=		/usr/ports
 LOCALBASE?=		/usr/local
 LINUXBASE?=		/compat/linux
 DISTDIR?=		${PORTSDIR}/distfiles
-_DISTDIR?=		${DISTDIR}/${DIST_SUBDIR}
+_DISTDIR?=		${DISTDIR}${DIST_SUBDIR:D/${DIST_SUBDIR}}
 INDEXDIR?=		${PORTSDIR}
 SRC_BASE?=		/usr/src
 USESDIR?=		${PORTSDIR}/Mk/Uses