hostname: when domain is none, it should be outputted to stdout

[Devel08]

GNU:

$ hostname --domain
(none)

uutils:

$ cargo run -q -p uu_hostname -- --domain

[xtqqczze]

Right, the literal (none) seems to be what is expected, at least for Linux:

#ifndef UTS_DOMAINNAME
#define UTS_DOMAINNAME "(none)"	/* set by setdomainname() */
#endif

• https://github.com/torvalds/linux/blob/6f8319e3e9a44dd537d17f41565a8453c560a581/include/linux/uts.h#L16-L18
• https://access.redhat.com/solutions/352963

[xtqqczze]

@collinfunk could GNU add a test?