rephrase wal to "write-ahead log" in cli

Many users might not know what "WAL" stands for.
This will make it more explicit.

util.c

@@ -99,15 +99,15 @@ usage(int code)
     fprintf(stderr, "Use: %s [OPTIONS]\n"
             "\n"
             "Options:\n"
-            " -b DIR   wal directory\n"
+            " -b DIR   write-ahead log directory\n"
             " -f MS    fsync at most once every MS milliseconds"
                        " (use -f0 for \"always fsync\")\n"
             " -F       never fsync (default)\n"
             " -l ADDR  listen on address (default is 0.0.0.0)\n"
             " -p PORT  listen on port (default is " Portdef ")\n"
             " -u USER  become user and group\n"
             " -z BYTES set the maximum job size in bytes (default is %d)\n"
-            " -s BYTES set the size of each wal file (default is %d)\n"
+            " -s BYTES set the size of each write-ahead log file (default is %d)\n"
             "            (will be rounded up to a multiple of 512 bytes)\n"
             " -c       compact the binlog (default)\n"
             " -n       do not compact the binlog\n"