`std::io::fs::mkdir_recursive` overhaul

[kud1ing]

• Currently std::io::fs::mkdir_recursive calls itself recursively. For deep paths this could (in theory) be a problem.
  An interative version could avoid this.
• Consider renaming it to something like make_dirs (analogous to make_dir)
• Deal with the case "directory exists but with different permissions".

[toddaaro]

@msullivan has some thoughts on this, pinging him to resolve it.

[huonw]

Triage visit; still relevant: same name, still recursive, and even has the following comment:

// tjc: if directory exists but with different permissions,
// should we return false?

os::mkdir_recursive needs a bit of an overhaul.

[kud1ing]

I've renamed and reworded the issue.

[sanxiyn]

Triage. Still recursive, permission comment still there, but moved to std::io::fs::mkdir_recursive.