Added support for monster spritename constants

- Follow-up to 124ab2a1cdb344f24170a4d91f7000ebabf39b40
- Replaced numeric IDs in NPC definitions and setnpcdisplay calls with
  the appropriate constants
- Made possible thanks to Ind

Signed-off-by: Haru <haru@dotalux.com>
Added mercenary/homunculus constants from Aegis

- Follow-up to 124ab2a1cdb344f24170a4d91f7000ebabf39b40
- Replaced last leftover numeric sprite ID with the appropriate constant
- Special thanks to Ind

Signed-off-by: Haru <haru@dotalux.com>