Free action bonus against grabs too high

The bonus given by free action to evade grab attacks
is too high (mhitu.c 873-879). The comment suggests
that a player with luck = 0 will have a 50% chance of
evading the grab, whereas in the code a 60% chance has
been given.

In combination with luck = 13 and a helmet, this 60%
chance gives mind flayers a negligible chance at a
mindflay (not to mention those poor eels and kraken).
One of the scariest monsters of vanilla is a bit of a
joke in Slash'EM.