log: invert the logging continuation and strip newlines

For all but the simplest loggers, the current approach of "this is a continuation of the previous message" doesn't work well. The caller cannot know whether the current message is complete until it receives the next message - but that message may never come.

Invert this logic, send a boolean flag to notify the caller that the current log message is not complete logical message and the caller should buffer this message until ready to print.

Plus, drop the trailing newlines and let the caller handle those.

Fixes #19 (closed)