Summary of changes:
* Make xvsnprintf() static
* restrict-qualify pointer arguments (as done by the libc equivalents)
* Make comments and spec references more thorough
* Remove pointless `n <= INT_MAX` assertion (see comment)
* Use FATAL_ERROR() instead of xassert() (since the assertion is inside
a shared util function but the caller is responsible for ensuring the
condition holds true)
* Change some callers to use size_t instead of int for the return value
(negative returns are impossible and all subsequent uses are size_t)
The updated comments and code were taken (and adapted) from:
49260bb154/src/util/xsnprintf.c (L6-50)
This work was entirely authored by me and I hereby license this
contribution under the MIT license (stated explicitly, so that
there's no ambiguity w.r.t. the original license).
