feat(nsh_cat): allow cat to read from stdin

Now, if we run cat without arguments, it will just read from stdin.

It can be used with redirect like `cat < infile > outfile`.

nshlib/nsh_command.c

@@ -158,8 +158,8 @@ static const struct cmdmap_s g_cmdmap[] =
 #endif
 
 #ifndef CONFIG_NSH_DISABLE_CAT
-  CMD_MAP("cat",      cmd_cat,      2, CONFIG_NSH_MAXARGUMENTS,
-    "<path> [<path> [<path> ...]]"),
+  CMD_MAP("cat",      cmd_cat,      1, CONFIG_NSH_MAXARGUMENTS,
+    "[<path> [<path> [<path> ...]]]"),
 #endif
 
 #ifndef CONFIG_DISABLE_ENVIRON

nshlib/nsh_fscmds.c

@@ -791,6 +791,25 @@ int cmd_cat(FAR struct nsh_vtbl_s *vtbl, int argc, FAR char **argv)
         }
     }
 
+  if (argc == 1)
+    {
+      char *buf = malloc(BUFSIZ);
+
+      /* Dump from input */
+
+      while (true)
+        {
+          ssize_t n = nsh_read(vtbl, buf, BUFSIZ);
+
+          if (n == 0)
+            break;
+
+          nsh_write(vtbl, buf, n);
+        }
+
+      free(buf);
+    }
+
   return ret;
 }
 #endif