hostname: Allow comments in /etc/hostname

The hostname(1) tool allows comments in /etc/hostname. Introduce a new
read_hostname_config() in hostname-util which reads a hostname configuration
file like /etc/hostname, strips out comments, whitespace, and cleans the
hostname. Use it in hostname-setup.c and hostnamed and remove duplicated code.

Update hostname manpage. Add tests.

https://launchpad.net/bugs/1053048