Make log_unix_epoch an official alias of log_alternate_epoch

Make log_unix_epoch an official alias of log_alternate_epoch

log_alternate_epoch was introduced along with
log_alternate_epoch_clock_id, and generalized the idea of
log_unix_epoch. Both options had the same effect. So we make
log_unix_epoch an official alias of log_alternate_epoch, instead of
maintaining both redundant options.

Signed-off-by: Nick Neumann nick@pcpartpicker.com