Fix a bug in the core files framework with handling the BOF buffer.

- Any files where the total size was below the size of the
   default bof_buffer size couldn't have stream analyzers successfully
   attached because the bof_buffer never reached the full size
   and was never flushed.  This branch explicitly marks the buf_buffer
   as full and flushes it when the file is being removed.

testing/btest/scripts/base/frameworks/file-analysis/big-bof-buffer.bro

@@ -0,0 +1,6 @@
+# @TEST-EXEC: bro -r $TRACES/http/get.trace %INPUT
+# @TEST-EXEC: btest-diff files.log
+
+@load frameworks/files/hash-all-files
+
+redef default_file_bof_buffer_size=5000;