cleanup: remove MemSet() casts

There were various locations in the code that were casting the first
argument to MemSet() or occasionally memset() to a char *. This is not
necessary, per C89: memset's first argument is a void * to begin with,
and there is an implicit conversion between void * and any other pointer
type.

I also noticed a bit of unclear / inconsistent code in MemSet() itself:
it's not obvious whether unary * or postfix ++ has higher precedence
(the latter does), so I added some parentheses to make this clear.

Barring any objections, I'll apply this to HEAD tomorrow.

-Neil