Modify default log timestamp pto conform to RFC 5424 format

the attached patch changes the default timestamp format in logfiles to conform to RFC 5424 (the current version of the syslog RFC). It is identical to the current default "ISO 8601" timestamp used by slurm, with the exception that the timezone offset is appended. This has the benefits of

1) It's unambiguous.

2) Avoids potential confusion for admins running cluster(s) in different timezones.

3) Might help debug issues related to DST transitions. (More on that later..)

(To be pedantic, a RFC 5424 timestamp is still a valid ISO 8601 timestamp, but the converse is not necessarily true. So there is RFC 3339 which is a "profile" of ISO 8601, that is a subset, recommended for internet protocols. The RFC 5424 timestamp, in turn, is a subset of the RFC 3339 timestamps.)

The previous behavior of can be used by running configure with the

--disable-rfc5424time

flag.