Make rotting-by-miasma gradual instead of instant.

Very few things can instantly rot the player - mutation while undead,
monsters touching the orb in ZotDef, and ... miasma. Considering the
nature of miasma and it being comparable to poison clouds in that it
inflicts negative effects that increase in magnitude as the player
continues to stand in the cloud, it seems odd that just by touching
miasma for a turn that you're going to instantly rot a bit on the
outside.

Hopefully, this should lessen uncontrollable player aggravation a bit.