tools/systemd/time: let libc normalize time for us

tools/systemd/time: let libc normalize time for us

mktime/gmtime can normalize time and even can handle special timezone
cases like the fact that the time 2:30 on specific day/timezone combos
do not exists

we have to convert the signature of all functions that use
normalize_time since mktime/gmtime can return an EOVERFLOW
but if this happens there is no way we can find a good time anyway

since normalize_time will always set wday according to the rest of the
time, remove set_wday

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>