Instead of checking for the *etcXX.tgz filename, check whether the

Instead of checking for the *etcXX.tgz filename, check whether the
tarball contains ./var/db/sysmerge/etcsum or ./var/db/sysmerge/xetcsum.
First, it's more correct and second it allows people to rename the sets
without sysmerge bailing out.

feature requested and tested by oga@