If the curl fails or receives a non-2xx status code, it retries, backing
off a little bit, until it either succeeds, or exceeds five attempts.

Adds support for --prop-file, --prop-url and --prop-name, which let one
augment the boot sequence of a ship with additional events.

Back-ports the vere changes from urbit/urbit#5470 onto current vere. See
that for additional details/history. This is a mostly straightforward,
copy-paste port of the changes from that original PR, with the following
exceptions:
- Boot sequence construction now happens in _pier_boot_make.
- We don't modify the way the default boot sequence is acquired (yet).

We will want to follow up on the latter, eventually.

The %autoprop "prop builder" that was included in #5470 should still
operate fine and be useful for automating keeping certain kinds of props
up to date. (Save for minimal kelvin compatability disrepair it has
fallen into due to the flow of time.)

For safety.