Fix ERROR 2 (...) in putnode.cpp ... opening "node.exb"

Need the O_CREAT mode flag.

Broken with commit d116f362: I started using opennodeext() (created 5 years
ago with commit 710bb23c) rather than nopen(), and I didn't realize that
opennodeext() was not including O_CREAT.

Once the ctrl/node.exb was created (by anything), this error wouldn't happen.
And it'd only be an issues for systems with text.dat's that included custom
node status (NodeAction*) strings. Good catch Nelgin!

src/sbbs3/userdat.c

@@ -1267,7 +1267,7 @@ int opennodeext(scfg_t* cfg)
 		return -1;
 
 	SAFEPRINTF(fname, "%snode.exb", cfg->ctrl_dir);
-	return nopen(fname, O_RDWR | O_DENYNONE);
+	return nopen(fname, O_CREAT | O_RDWR | O_DENYNONE);
 }
 
 /****************************************************************************/