[RDY] 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?
Small corrections:
```c
/// Reads a string of length "cnt" from "fd" into allocated memory.
/// @return pointer to the string or NULL when unable to read that many bytes.
```
ref. http://neovim.org/develop/style-guide.xml#Function_Comments