[urbit] New and improved startup sequence

[Raymond Pasco]

Urbit master now has a new and better-behaved startup sequence that doesn't
assume it's running out of its own source distribution directory. Changes:

- urbit.pill now fetches itself from the latest into your pier directory
when creating a new pier. To override this manually, use the -B option to
specify a pill file. This is an esoteric developer option; don't manually
download the pill at all, ever, unless your automatic download fails.
- To facilitate the above, libcurl is now a dependency.
- When starting a galaxy, instead of a hardcoded path, the location of the
initial arvo sync files is now specified with the -A option.

We're very close now to an "urbit 0.1" release that you can install from
your system package manager like any normal socially responsible Unix
software.

Please test the pill fetching on your platform - just start a fake ~zod.
Remember the new -A option that is required to start galaxies!

Yours,
r