Use 'make show' on OpenBSD when applicable

'-V foo -V bar' doesn't yield the expected results (nor does it
always expand variables) for multiple variables, so fall back to
it for single variables.
Move FreeBSD specific variables into the 'freebsd' block
Don't treat failing to read MOVED as an error for it doesn't exist on OpenBSD
Add 'node' to prefixes
use try/catch for creating URI objects

a single port with a failure shouldn't kill the entire run
add some xz related extensions
Add new config option (mastersite ignore) to prevent recording backup mirrors
Add .tar.xz too
Add some more *.Z extensions