hostname: implement -F FILE

[omnivagant]

or, from net-tools hostname(1):

-F, --file filename
              Read the new host name from the specified file. Comments (lines
              starting with a `#') are ignored.

I'm not sure if this is out-of-scope, since I don't think GNU Coreutils hostname has it, but hostname provided by both busybox and net-tools (also has long flag --file) do.

In Alpinelinux , I keep the hostname symlink in a separate subpackage, uutils-hostname, due to this, and it looks like we don't install it for our GNU Coreutils package.

I think we, at least used to, depend on the -F flag, but right now I cannot find where

[oech3]

https://github.com/uutils/hostname ?