hostname: Add an implementation of the hostname display utility.

[Juan Lang]

Hi Andrew,

+    GetComputerNameW(name, &size);
+    hostname_printfW(fmtW, name);

I'm pretty sure that's not what you want.  GetComputerName returns the
"NetBIOS name" of the computer, which is set in the registry.
hostname prints the IP hostname of the computer.  At least, my look at
a Windows version's imports showed no import of GetComputerName, while
there is an import of ws2_32.dll's gethostname (by ordinal).
--Juan