zdev: Improve handling of invalid udev rules

Add basic checking when reading udev rules and print a warning when an
invalid udev rule file is found. This addresses the current inconsistent
behavior:

  - CCW devices: invalid udev rules are considered valid
  - CCW group devices: invalid udev rules are silently ignored
  - zFCP LUNs: invalid rules cause chzdev/lszdev to silently terminate
    with non-zero exit code

Checks include a test for empty or truncated rule files that might be
the result of an interrupted chzdev operation, or a file system or I/O
error.

Note: The recommended way to correct invalid udev rules is to either
remove the offending rules file, or to repeat the associated persistent
configuration step.

Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reported-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

zdev/include/udev.h

@@ -40,6 +40,7 @@ struct udev_file {
 };
 
 exit_code_t udev_read_file(const char *, struct udev_file **);
+bool udev_file_is_empty(struct udev_file *file);
 void udev_free_file(struct udev_file *);
 void udev_file_print(struct udev_file *);