multishell - work around remote+sudo+homedir problem, tidy shell resolution
- Work around the shell/tramp mishandling of remote+sudo+homedir problem!
The work around is clean and simple, basically using high-level `cd'
API and not messing with the low-level default-directory setting.
(Turns out the problem was not in my local config. Good riddance to the
awkward failure handler!)
- Clean up code resolving the destination shell, starting to document the
decision tree in the process. See getting-to-a-shell.md in the
multishell repository, https://github.com/kenmanheimer/EmacsMultishell