Make timezone setting easier

[thommey]

Right now we use $offset and $timezone and set that into the TZ env variable instead of just letting users set env(TZ) to Europe/Berlin and similar. I don't know if $offset and $timezone are used in scripts. if they are, we could figure them out from the env(TZ) setting.
I see little reason to allow people to set offset/timezone only but not the shell environment timezone that affects everything eggdrop does.