MINOR: log-format: allow to preserve spacing in log format strings

MINOR: log-format: allow to preserve spacing in log format strings

Now it's possible to preserve spacing everywhere except in "log-format",
"log-format-sd" and "unique-id-format" directives, where spaces are
delimiters and are merged. That may be useful when the response payload
is specified as a log format string by "lf-file" or "lf-string", or even
for headers or anything else.

In order to merge spaces, a new option LOG_OPT_MERGE_SPACES is applied
exclusively on options passed to function parse_logformat_string().

This patch fixes an issue #701 ("http-request return log-format file
evaluation altering spacing of ASCII output/art").