Fix ddb rc script

r288291 added a call to limits(1), which isn't available before partitions
are mounted. This broke the ddb rc script, which does not provide its own
start_cmd.

Alleviate the situation here by providing a start_cmd. We still have other
problems with diskless setups that need to be considered, but this is a
start.

PR: 206291
Submitted by: cy
Discussed with: rgrimes
MFC after: 3 days