std: implement hostname

[joboet]

Resolves rust-lang/libs-team#330
Tracking issue: #135142

This is based on #135141, but I've reimplemented the UNIX version, which now:

• uses sysconf(_SC_HOST_NAME_MAX) as an initial buffer length
• returns OutOfMemory if the Vec allocation fails
• retries the operation if it detects that the name returned by gethostname was truncated

Additionally, as part of the rebase, I had to move some WinSock abstractions (initialisation and error access) to sys::pal so that they can be accessed from sys::net::hostname.

CC @orowith2os (and thank you for your work!)