libsa: Add a generic URI-syntax devspec parser

[imp]

Summary

Add parse_uri(), a helper for devices whose devspec can take a
"[N]://host[:port][/path]" form in addition to the traditional "N:path"
form. It returns EINVAL when devspec doesn't match that pattern, so a
caller's own dv_parsedev can fall back to the traditional form.

Un-static default_parsedev() so it can serve as that fallback.

[imp]

rebase / update