DIET/MINOR: listener: rearrange a few fields in struct listener to save 16 bytes

DIET/MINOR: listener: rearrange a few fields in struct listener to save 16 bytes

Pack the listener state to 1 char, store it as an enum instead of an
int (more gdb-friendly), and move a few fields around to fill holes.

The <nice> field can only be -1024..1024 so it was stored as a signed
short and completes well with obj_type and li_state.

Doing this has reduced the struct listener from 376 to 360 bytes (4.2%).