Merge of r4636, r4637, r4638: config sanity checks.

Merge of r4636, r4637, r4638: config sanity checks.

*) Added syntax checking of the second parameter of the "split_clients"
   directive.

*) Capped the status code that may be returned with "return" and
   "try_files".

*) Zero padded the returned and logged HTTP status code, and fixed possible
   buffer overrun in $status handling.