File extraction: use fseek

In the past, we allocated a buffer with zeroes and wrote that with
fwrite. Now, instead we just fseek to the correct offset.

This changes the way in which the file extract limit is counted a bit;
skipped bytes do no longer count against the file size limit.

(cherry picked from commit 5071592e9b7105090a1d9de19689c499070749d4)

src/file_analysis/analyzer/extract/Extract.h

@@ -72,8 +72,8 @@ protected:
 private:
 	std::string filename;
 	FILE* file_stream;
-	uint64_t limit;
-	uint64_t depth;
+	uint64_t limit; // the file extraction limit
+	uint64_t written; // how many bytes we have written so far
 	};
 
 	} // namespace zeek::file_analysis::detail