Revert "main/dhcpcd: upgrade to 6.11.0"

This reverts commit 0587d55d.

The new release seems to have major changes, and is buggy. See:
http://roy.marples.name/projects/dhcpcd/info/05a4883e1efe588e
http://roy.marples.name/projects/dhcpcd/info/3f10c9b871b78236

Reverting for now until the above bugs are fixed.