[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?
My original reasoning for using uint8_t was that we are returning bytes from a file, and in most systems bytes are probably going to be 8 unsigned bits.

But the method is actually an interface to get a string, so it is more accurate to return characters from it. I agree that is correct. :) The caller is going to assume they are going to get a string back. Since the data type used to read the characters is unsigned char, as per the man page for getc, I realized that it is possible, though very unlikely, that the value returned by getc may not fit in uint8_t at some point in the future. That is why I changed everything to char. It is guaranteed to fit the data, and the representation will be the same when the data is used by the caller.