This was horribly broken:
* pack_key8() and pack_key16() ...
  * missed to invert retain value when applying it to the mask,
  * did not sanitize val by ANDing it with retain,
  * and ignored the mask which is necessary for 'invert' command.
* pack_key16() did not convert mask to network byte order.
* Changing the retain value for 'invert' or 'retain' operation seems
  just plain wrong.
* While here, also got rid of unnecessary offset sanitization in
  pack_key32().
* Simplify code a bit by always assigning the local mask variable to
  tkey->mask before calling any of the pack_key*() variants.
Signed-off-by: Phil Sutter <phil@nwl.cc>