argv: permit repeated flags

This commit builds on the previous argv refactor by being more principled
about how we declared our flags. In particular, we now require that every
clap argument is one of three things: a positional argument, a switch or
a flag that accepts exactly one value. The latter two are always permitted
to be repeated, and we modify the code that consumes a clap::ArgMatches to
always use the *last* value of an argument. (clap by default always uses
the first value of argument, if it has been repeated and is accessed via
one of the singleton accessors.)

Fixes #553