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!