basic: nicer xsprintf and xstrftime assert messages

It's nicer if the assertion failure message from a bad use of xsprintf
actually mentions xsprintf instead of the expression the macro is
implemented as.

The assert_message_se macro was added in the previous commit as an
internal helper, but it can also be used for customizing assertion
failure messages like in this case.

Example:
  char buf[10];
  xsprintf(buf, "This is a %s message.\n", "long");

Before:
  Assertion '(size_t) snprintf(buf, ELEMENTSOF(buf), "This is a %s
  message.\n", "long") < ELEMENTSOF(buf)' failed at foo.c:6, function
  main(). Aborting.

After:
  Assertion 'xsprintf: buf[] must be big enough' failed at foo.c:6,
  function main(). Aborting.