Transition to a more versatile wrapper

[xparq]

Follow-up of #6

• Approximate the missing var="val" cmd feature of sh!
• Set vars from a (.profile-like) config file.
  • The killer feature here is a good set of default locations scanned, and a natural, simple, but powerful, extendable syntax for the config file!
    • Plus sane contexts (project) nesting (-> learn from the pain points of e.g. git submodules)... This might just mean warning + subshell, or even bail-out. And local vars overriding the outer ones, as usual. (Plus scoping.) See also other Win-specific vars + my regenv tool!
      -> Take the (good) features of p-box; even be a drop-in replacement for it!
• Path conversions for the arguments and also (selected) env vars... (E.g. to support uniform path inputs for different toolchains from makefiles.)
• Keep the syntax cross-platform!
• Rename to w, for a start...
• But avoid accidentally becoming a new shell! :)