Modify mount_propagation_flags_from_string to return a normal int code

This means that callers can distiguish an error from flags==0,
and don't have to special-case the empty string.

src/basic/mount-util.h

@@ -63,4 +63,4 @@ int mount_verbose(
 int umount_verbose(const char *where);
 
 const char *mount_propagation_flags_to_string(unsigned long flags);
-unsigned long mount_propagation_flags_from_string(const char *name);
+int mount_propagation_flags_from_string(const char *name, unsigned long *ret);