fix rare replication stream corruption with disk-based replication

The slave sends \n keepalive messages to the master while parsing the rdb,
and later sends REPLCONF ACK once a second. rarely, the master recives both
a linefeed char and a REPLCONF in the same read, \n*3\r\n$8\r\nREPLCONF\r\n...
and it tries to trim two chars (\r\n) from the query buffer,
trimming the '*' from *3\r\n$8\r\nREPLCONF\r\n...

then the master tries to process a command starting with '3' and replies to
the slave a bunch of -ERR and one +OK.
although the slave silently ignores these (prints a log message), this corrupts
the replication offset at the slave since the slave increases the replication
offset, and the master did not.

other than the fix in processInlineBuffer, i did several other improvments
while hunting this very rare bug.

- when redis replies with "unknown command" it includes a portion of the
  arguments, not just the command name. so it would be easier to understand
  what was recived, in my case, on the slave side,  it was -ERR, but
  the "arguments" were the interesting part (containing info on the error).
- about a year ago i added code in addReplyErrorLength to print the error to
  the log in case of a reply to master (since this string isn't actually
  trasmitted to the master), now changed that block to print a similar log
  message to indicate an error being sent from the master to the slave.
  note that the slave is marked as CLIENT_SLAVE only after PSYNC was received,
  so this will not cause any harm for REPLCONF, and will only indicate problems
  that are gonna corrupt the replication stream anyway.
- two places were c->reply was emptied, and i wanted to reset sentlen
  this is a precaution (i did not actually see such a problem), since a
  non-zero sentlen will cause corruption to be transmitted on the socket.

src/server.c

@@ -2345,8 +2345,13 @@ int processCommand(client *c) {
     c->cmd = c->lastcmd = lookupCommand(c->argv[0]->ptr);
     if (!c->cmd) {
         flagTransaction(c);
-        addReplyErrorFormat(c,"unknown command '%s'",
-            (char*)c->argv[0]->ptr);
+        sds args = sdsempty();
+        int i;
+        for (i=1; i < c->argc && sdslen(args) < 128; i++)
+            args = sdscatprintf(args, "`%.*s`, ", 128-(int)sdslen(args), (char*)c->argv[i]->ptr);
+        addReplyErrorFormat(c,"unknown command `%s`, with args beginning with: %s",
+            (char*)c->argv[0]->ptr, args);
+        sdsfree(args);
         return C_OK;
     } else if ((c->cmd->arity > 0 && c->cmd->arity != c->argc) ||
                (c->argc < -c->cmd->arity)) {