PWMAngband: aquatic equilibrium

– Fix damaging terrain aborting running for players immune to damage
– Fix aquatic forms able to run outside of water
– Fix aquatic forms not suffocating outside of water
– Fix aquatic forms drowning in water
– Fix LEVITATE form not giving FEATHER flag
– Fix FRIGHTENED form not giving AFRAID flag
– Fix Linux code not saving savefiles
– Re-enable signal handling on Linux