Change how current emerge time is calculated a bit

Change how current emerge time is calculated a bit

Instead of having some ceiling value for the emerge time (like it was
previously 12*3600 seconds) calculate current emerge time from each of
the entries in the log and choose the smallet value. This should
always be the last entry on the log and therefore the one that was
started closest to the current time.

In other words; the current emerge mode now works for all those
countless packages that take days compile.