logprint: implement log_print_log_line()

logprint: implement log_print_log_line()

A brand new, clean room implementation of log_print_log_line()
made with the specification consisting of the existing tests,
and a bit of the other existing code, but not the old implementation
of log_print_log_line().

There was one additional change: "inline warning". Initially it was printed
sometimes with its own copy of log metadata, sometimes without it.
Now, it's consistent with the printing format: the warning gets
its own metadata only if separate lines in logs get their own metadata.

Change-Id: I93f3b7864854a6a988a9ae02743df54ad46a37fe