MEDIUM: log-format: relax parsing of '%' followed by unsupported characters

MEDIUM: log-format: relax parsing of '%' followed by unsupported characters

At the moment when a '%' character is followed by any unhandled character,
it is considered as a variable name, and if it cannot be resolved, a warning
is emitted and the configuration goes on.

When we start using log-format for redirect rules, it may happen that some
people accidently use '%' instead of '%%' without understanding the cause
of the issue. Thus we do two things here :

   - if a single '%' is followed by a blank or a digit, we fix it and emit a
     warning explaining how this should be done ; this ensures that existing
     configs continue to work ;

   - if a single '%' is followed by an unknown variable name, we report it
     and explain how to emit a verbatim '%' in case this is what the user
     desired.