os: add get_path_env function

[aturon]

Parsing the $PATH environment variable is nontrivial (not just a simple separator split) and is platform dependent. The std::os module should provide

fn get_path_env() -> Vec<Path>

that does the appropriate parsing. See #11324 for a starting point, and consult http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html for some guidance. Note that Windows has its own conventions.