syslogd: Avoid trimming host names in RFC 5424 mode

RFC 5424 says that implementations should log hostnames in FQDN
format. Only trim host names in RFC 3164 mode.

PR: 250014
Submitted by: Dmitry Wagin <dmitry.wagin@ya.ru>
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D26644