miniupnpd/Makefile: allow to override FWNAME
miniupnpd/bsd/getifstats.c: fix for DragonFly BSD

struct ifnet is private to kernel code and is hidden from the userland
code unless you define a macro _KERNEL_STRUCTURES.  Although other people
argue that it should be exposed as it is on other BSDs, that's the way
it is on DragonFly BSD.

Thanks to YONETANI Tomokazu for DragonFly BSD patches