Fix fcntl F_GETFL F_SETFL for files opened execute-only (O_EXEC).

The FFLAGS and OFLAGS now work correctly also for files opened with O_EXEC.
Except possibly fuse, the other users pass values without O_EXEC set. fuse
appears to assume O_EXEC is handled correctly.

Although F_SETFL may not be commonly used for execute-only file descriptors,
F_GETFL may be useful to find the access mode.