+is_player_slimeable(), is_monster_slimeable()

Code refactoring.

Fixes bug that biting slime was setting counter to 10, also for slimed characters - so it was potential improvement of slimy situation. It was not affecting vampires as they are unable to drain green slimes.

For unknown reason not all flaming moster were sliming resistant via biting.

Prepares ground for C343-81 bugfix.

inspired by https://github.com/sgrunt/GruntHack/commit/53922d0e37b65a3ec7379b65e9d4c7e2ea4c36ce