This will enable us to query for the current system's hostname in both
Unix and Windows environments.
We could have pulled in the 'gethostname' crate for this, but:
1. I'm not a huge fan of micro-crates.
2. The 'gethostname' crate panics if an error occurs. (Which, to be
fair, an error should never occur, but it seems plausible on borked
systems? ripgrep runs in a lot of places, so I'd rather not take the
chance of a panic bringing down ripgrep for an optional convenience
feature.)
3. The 'gethostname' crate uses the 'windows-targets' crate from
Microsoft. This is arguably the "right" thing to do, but ripgrep
doesn't use them yet and they appear high-churn.
So I just added a safe wrapper to do this to winapi-util[1] and then
inlined the Unix version here. This brings in no extra dependencies and
the routine is fallible so that callers can recover from potentially
strange failures.
[1]: https://github.com/BurntSushi/winapi-util/pull/14
