cli: add new 'hostname' function

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

crates/cli/src/lib.rs

@@ -144,6 +144,7 @@ error message is crafted that typically tells the user how to fix the problem.
 
 mod decompress;
 mod escape;
+mod hostname;
 mod human;
 mod pattern;
 mod process;
@@ -155,6 +156,7 @@ pub use crate::{
         DecompressionReader, DecompressionReaderBuilder,
     },
     escape::{escape, escape_os, unescape, unescape_os},
+    hostname::hostname,
     human::{parse_human_readable_size, ParseSizeError},
     pattern::{
         pattern_from_bytes, pattern_from_os, patterns_from_path,