Use opennodedat() always to open the node.dab file

Include O_CREAT access mode flag in opennodedat().

I experimented with excluding O_DENYNONE when the NM_CLOSENODEDAB flag is set
(to hopefully work-around MacOS Samba node.dab corruption issue), but that
didn't work with SBBSCTRL leaving the file open in SH_DENYNONE mode, so will
have revist that, but using a common open function helps.