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).

terminal.c

@@ -4302,7 +4302,7 @@ term_send_size_notification(struct terminal *term)
     const int width = term->width - term->margins.left - term->margins.right;
 
     char buf[128];
-    const int n = xsnprintf(
+    const size_t n = xsnprintf(
         buf, sizeof(buf), "\033[48;%d;%d;%d;%dt",
         term->rows, term->cols, height, width);
     term_to_slave(term, buf, n);