Fringe cases 2 (cont): errors

some archs don't build some things
mono & avahi
dpb errors because some stuff doesn't happen
double builds of avahi vs avahi,no_mono
new <bsd.port.arch.mk>
takes MULTI_PACKAGES
handles PSEUDO_FLAVORS
handles ONLY_FOR_ARCHS-mono
yields BUILD_PACKAGES: the real thing
helps dpb
simpler for normal builds