Use FQDN in RFC5424 Layout (+review #2853)

Since RFC5424 specifies that the
[`HOSTNAME`](https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.4)
field should contain the FQDN of the current host,
we replace replace the usage of `InetAddress.getHostName()`
with `InetAddress.getCanonicalHostName()`.

Closes #1740.
7 files changed