cli: intercept_state.go: Clean up arg parsing
cli: intercept_state.go: Clean up arg parsing

- Rename 'interceptInfo' to 'interceptArgs' and have it be *just* a
  plain struct of the parsed flags and arguments.

- Create a separate anonymouse RunE wrapper to handle the arg parsing,
  separate the parsing from the code of actually creating the
  intercept.

- Move loginIfNeeded out to be a proper top-level function, rather
  than a closure.

All in all, this is pretty mundane and is a lot of noise with adding
'.args' or '.cs' in to a bunch of accesses on interceptState.

Signed-off-by: Luke Shumaker <1abe3165bcc8c29e8b31e5c88636540fb9350004@datawire.io>

datawire/telepresence,datawire/telepresence,datawire/telepresence