MINOR: log: print metadata prefixes separately in sess_build_logline()

MINOR: log: print metadata prefixes separately in sess_build_logline()

Some log variables may be prefixed with specific chars that represent
extra informations that are relevant with it but are are not directly
part of the "raw" value.

ie: '+' char is prepended before some values when "option logasap" is
used to indicate that the value has not yet reached its final value.

However, as those "metadata" are printed using the general purpose
LOGCHAR() printing helper, it's not easy to tell if they are part of the
base value or not.

In this patch we add the LOGMETACHAR() helper that is a wrapper for
LOGCHAR(). The goal is to prepare for adding some logic to prevent such
additional infos from being generated when not relevant or needed.