Update branch EoLs:

• Add entry for RELENG_6_1. This is an Extended branch, and will expire

at the end of May 2008.

• Update EoL of RELENG_6: The branch will be alive at least as long as

RELENG_6_1 (and longer, assuming there are more 6.x releases).

• Extend EoL of RELENG_5_4 to match the EoL of RELENG_5_3 (October 2006).

This will give users of FreeBSD 5.4 more time to upgrade to 5.5 (which
will be released very soon).

• Update EoL of RELENG_5: It will die at the same time as RELENG_5_5,

which will be the end of May 2008.