MEDIUM: log-format: make the format parser more robust and more extensible

MEDIUM: log-format: make the format parser more robust and more extensible

The log-format parser reached a limit making it hard to add new features.
It also suffers from a weak handling of certain incorrect corner cases,
for example "%{foo}" is emitted as a litteral while syntactically it's an
argument to no variable. Also the argument parser had to redo some of the
job with some cases causing minor memory leaks (eg: ignored args).

This work aims at improving the situation so that slightly better reporting
is possible and that it becomes possible to extend the log format. The code
has a few more states but looks significantly simpler. The parser is now
capable of reporting ignored arguments and truncated lines.