xsnprintf: various improvements related to xvsnprintf() and xsnprintf()

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).

meson.build

@@ -216,6 +216,7 @@ common = static_library(
   'log.c', 'log.h',
   'char32.c', 'char32.h',
   'debug.c', 'debug.h',
+  'macros.h',
   'xmalloc.c', 'xmalloc.h',
   'xsnprintf.c', 'xsnprintf.h'
 )