Add "Auto" Local Time Zone option and make that the default for new install

As Deon pointed out in DOVE-Net / Synchronet Discussion, having a local time
zone configured with a different UTC offset than your system time zone can
produce strange/unexpected results (e.g. displayed age of messages). Since
it's possible that not all sysops will complete the configuration wizard or
actually set their timezone to the correct value (and ignore the startup
warning message), we now make the default Local Time Zone to be
"automatic" - query the OS every time the local time zone is needed/used.

This has the downside of only storing (e.g. in message headers) the UTC
offset of the current time zone (not the time zone abbreviation/name as
encoded by SMB). I considered making an option to dynamically figure out
the actual time zone (not just the UTC offset) and while I think that's
doable, Deon just wanted his UTC offset (e.g. UTC+11:00) and not his
time zone name (e.g. AEDT) stored in message headers, so this setting
w...