LogFormat: add partial support for literal strings in logfiles. Limited to data prefi...

LogFormat: add partial support for literal strings in logfiles. Limited to data prefixes like "field=%field"

Reason:
Logging many fields results in very long log lines which in turn causes in very
unreadable log files. Visual cues in form of prefixes which describe data may
be used in log files. This commit adds support for ignoring such prefixes.

Why suffixes are not supported?
Awstats has data fields naming defined as %fieldName, but does not have all fields
joined in one place. This makes it nigh impossible to match, for example, this
specification: "dataOut=1024b" (note the "b" suffix which means data out is
measured in bytes). LogFormat specification for this would be "bytesOut=%bytesdb",
but in this case %bytesdb part would fail to parse out bytes transferred, as
awstats would look for field name called "bytesdb" instead of field name "%bytesd"
with "b" suffix denoting units of measurement.