Switch name check

[rjbou]

• Switch names (global or local directories) containing | '<' | '>' | '!' | '<backquote>' | '$' | '(' | ')' | ':', or empty are rejected. For windows compatibility, switch names that begin with \w:/ & \w:\ are allowed. Empty switche names are forbidden.
  • Compliance regexp is ^(\w:/\\)?[^<>!$():]+$ (missing the backquote because of markdown) cf. code
• Update opam installer to not use an empty switch to retrieve needed paths
• Cherry-picked context-free switch paths from Remove dependency on state value for opam2nix use cases #4147
• fix opam env OPAMSWITCH setting: if empty string, consider it as unset

fix #4231

@rjbou edit: update content

[rjbou]

Appveyor failure lightened something: should we allow local switches at system root, e.g. / or C:\?