[RFC] misc2.c -Wconversion changes

First go at helping out. Hopefully misc2.c goes away, but in the mean time I fixed up the one method in it that had -Wconversion errors, per #567.

xmallocz zeroes the string at the end, no need to double up on nulling it.

After looking, it seemed like a cnt of int was adequate, no callers used larger sizes. Still, should it be size_t?
`char_u` is the same as `uint8_t`. If this type is going to change it should be changed to `char`.