MINOR: log: add log profile buildlines

MINOR: log: add log profile buildlines

Now that we have log-profile parsing done, let's prepare for runtime
log-profile handling by adding the necessary string buffer required to
re-build log strings using sess_build_logline() on the fly without
altering regular loglines content.

Indeed, since a different log-profile may (or may not) be specified for
each logger, we must keep the original string and only rebuild a custom
one when required for the current logger (according to the selected log-
profile).