std::os: Add get_path_env()

[klutzy]

This primary motivation is that there was no easy way to get list of
paths from getenv("PATH"), because it requires to split string with
os-depdent separator.
rustc also did same work for RUST_PATH, so this patch moves the
portion into std::os for public use.

[alexcrichton]

I'm a little unsure of how serious we want to get with this, but I know that with libuv they attempt to parse the path on windows as well, and apparently it's not as easy as just splitting on semicolons (right now their path parsing causes an infinite loop).

If we do take this route, I fear that we'll eventually get a bug reporting saying that we're not parsing the windows paths correctly.

One route to take would be to only define the separator in libstd (which I think is done in std::path), and another route would be true path parsing for windows (I'm unsure if unix has the same ability). What do you think?

[klutzy]

Ugh windows! I presumed simple split works because gcc does for LIBRARY_PATH parsing, but it seems that PATH is more complicated.
posix says PATH is colon-separated so I think it's safe for unix.
On Windows, it seems that there is no concrete documentation of its syntax. libuv needs full emulation because they have to do it, but we don't. I started to think this patch doesn't provide good use cases to require such effort, so I'm closing this unless good reason is found. :-)

On the other hand, we may have to clarify what RUST_PATH syntax is. It is currently different to (and better than) PATH syntax on Windows.