Promote nodes set with --set to sequence nodes as needed.

A node isn't known to be a sequence node until the YAML is parsed.
If a node sequence node was set on the command line, promote
it to a sequence node when it is discovered by YAML to be
a sequence node.

Fixes comment #18 in issue 921.
eve-log: add warning if enabled but not supported

If we're not compiled against libjansson, the eve-log output is not
available. This patch adds a warning.