Make some fields ?= so it can become a master-port for for example

databases/db3-tcl

PR:		ports/60473 (related)

databases/db3/Makefile

@@ -17,8 +17,8 @@ PATCH_SITES=	http://www.sleepycat.com/update/${PORTVERSION}/
 PATCHFILES=	patch.${PORTVERSION}.1 patch.${PORTVERSION}.2
 PATCH_DIST_STRIP=	-d ${WRKDIR}/${DISTNAME}
 
-MAINTAINER=	matthias.andree@gmx.de
-COMMENT=	The Berkeley DB package, revision 3.3
+MAINTAINER?=	matthias.andree@gmx.de
+COMMENT?=	The Berkeley DB package, revision 3.3
 
 WRKSRC=		${WRKDIR}/${DISTNAME}/build_unix