[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?
Then you could create a `READ_STRING` macro that casts the return value of `read_string` and use it in place of `read_string` until all code can migrate from `char_u` to `char`.