journald: turn ForwardToSyslog= off by default

After all, rsyslog and friends nowadays read their data directly from
the journal, hence the forwarding is unnecessary in most cases.
hostnamed: minor modernization
nss-myhostname: simplify array building a bit
hostnamed: make use of in_charset() to verify charset
hostnamed: drop nss-myhostname check

The check only cares about whether the module is installed, not enabled.
But installation we should know anyway, after all we ship the module
with systemd these days...