bsd.port.mk: Implement BROKEN_${OPSYS} and BROKEN_${OPSYS}_${OSREL:R}

Examples of use:

• BROKEN_FreeBSD= does not link
• BROKEN_DragonFly= requires later jail
• BROKEN_FreeBSD_8= long type-name is invalid

The latter example could replace something like:

.include <bsd.port.pre.mk>
.if ${OPSYS} == FreeBSD && ${OSVERSION} <= 900000
BROKEN=	long type-name is invalid
.endif

Differential Revision: https://reviews.freebsd.org/D2207
Reviewed by: portmgr
Approved by: portmgr (mat)