implement our own assert() function

I recently upgraded the software on my macOS server and was dismayed to
see that cppcheck reported a huge number of format string errors due to
mismatches between the format string and its arguments from calls to
`assert()`. It turns out they are due to the macOS header using `%lu`
for the line number which is obviously wrong since it is using the C
preprocessor `__LINE__` symbol which evaluates to a signed int.

I also noticed that the macOS implementation writes to stdout, rather
than stderr. It also uses `printf()` which can be a problem on some
platforms if the stream is already in wide mode which is the normal case
for fish.

So implement our own `assert()` implementation. This also eliminates
double-negative warnings that we get from some of our calls to
`assert()` on some platforms by oclint.

Also reimplement the `DIE()` macro in terms of our internal
implementation.

Rewrite `assert(0 && msg)` statements to `DIE(msg)` for clarity and to
eliminate oclint warnings about constant expressions.

Fixes #3276, albeit not in the fashion I originally envisioned.

src/fallback.cpp

@@ -7,7 +7,6 @@
 
 // IWYU likes to recommend adding term.h when we want ncurses.h.
 // IWYU pragma: no_include term.h
-#include <assert.h>  // IWYU pragma: keep
 #include <dirent.h>  // IWYU pragma: keep
 #include <errno.h>   // IWYU pragma: keep
 #include <fcntl.h>   // IWYU pragma: keep