add xzrealloc

[elliefm]

Adds an xzrealloc() function that's like xrealloc() but also zeroes out the newly allocated bytes if the allocation grew in size. n.b. It needs one more argument than regular xrealloc(), so it's not a drop-in replacement.

[rsto]

Just found one nit. And this PR again higlights how much code still uses int at places where we really want to use size_t 😔

[ksmurchison]

Need to remove the stray memset() as pointed out by rsto. Once done, I approve without a re-review

requested changes applied; re-review not required