eve-http: print <unknown> like in eve-files

When UA or Host are unknown, print <unknown> instead of <useragent
unknown> or <hostname unknown>.

Bug #1131.
eve-files: file -> fileinfo

Due to what appears to be an issue in logstash, the 'file' part of
the file event types was masked by a field that logstash-forwarder
added itself.

Since logstash-forwarder is an important part of the logstash stack,
this patch works around the issue by renaming our 'file' structure
to 'fileinfo', thus resolving the naming conflict.

Bug #1127