BUG/MEDIUM: log: fix regression on log-format handling

Commit a4312fa2 merged into dev18 improved log-format management by
processing "log-format" and "unique-id-format" where they were declared,
so that the faulty args could be reported with their correct line numbers.

Unfortunately, the log-format parser considers the proxy mode (TCP/HTTP)
and now if the directive is set before the "mode" statement, it can be
rejected and report warnings.

So we really need to parse these directives at the end of a section at
least. Right now we do not have an "end of section" event, so we need
to store the file name and line number for each of these directives,
and take care of them at the end.

One of the benefits is that now the line numbers can be inherited from
the line passing "option httplog" even if it's in a defaults section.

Future improvements should be performed to report line numbers in every
log-format processed by the parser.
MEDIUM: log: report file name, line number, and directive name with log-format errors

Improve error log reporting in the format parser by always giving the
file name, line number, and directive name instead of the hard-coded
"log-format".

Previously we got this when dealing with log-format errors:
  [WARNING] 101/183012 (8561) : Warning: log-format variable name 'r' is not suited to HTTP mode
  [WARNING] 101/183012 (8561) : log-format: sample fetch <hdr(a,1)> may not be reliably used with 'log-format' because it needs 'HTTP request headers,HTTP response headers' which is not available here.
  [WARNING] 101/183012 (8561) : Warning: no such variable name 'k' in log-format

Now we have this :
  [WARNING] 101/183016 (8593) : parsing [fmt.cfg:8] : 'log-format' variable name 'r' is reserved for HTTP mode
  [WARNING] 101/183016 (8593) : parsing [fmt.cfg:8] : 'log-format' : sample fetch <hdr(a,1)> may not be reliably used here because it needs 'HTTP request headers,HTTP response headers' which is not available here.
  [WARNING] 101/183016 (8593) : parsing [fmt.cfg:15] : no such variable name 'k' in 'unique-id-format'