Switch tar(1) write default format to 'pax'

Switch tar(1) write default format to 'pax'

Lets us store longer file names, link names, finer grained timestamps,
larger archive member files, etc; at the expense of larger uncompressed
archives and less widespread support across the ecosystem.  If you're
unhappy with the new defaults, you can use -F ustar.  Or you can help
fix bugs / find a better middle ground.

Prodding from various including job@ and deraadt@
ok sthen@ caspar@ millert@