Added output plugin for unified2 files.

In some cases a user may need to run multiple snort processes, each of them
outputting to different unified logs, though they want to maintain a single
barnyard instance for final logging (i.e., multiple snorts with a single
barnyard2 mysql connection).

This output plugin will allow multiple by instances to output to a single
unified2 file (file locking is included so no race conditions happen). This
allows for another by2 instance to read from this aggregated unified2 log and
output to some other plugin.

It should be noted that the generated files using epoch timestamp are rounded
down to the hour so that two instances of by2 will always write to the same
file.