Replace assert(false) with an unreachable() macro.

Taking a leaf out of the LLVM code base: this macro still includes an
assert(false) so that the message will show up in a typical build, but
it follows it up with a call to a function explicitly marked as no-
return.

So this ought to do a better job of convincing compilers that once a
code path hits this function it _really doesn't_ have to still faff
about with making up a bogus return value or filling in a variable
that 'might be used uninitialised' in the following code that won't be
reached anyway.

I've gone through the existing code looking for the assert(false) /
assert(0) idiom and replaced all the ones I found with the new macro,
which also meant I could remove a few pointless return statements and
variable initialisations that I'd already had to put in to placate
compiler front ends.

nocmdline.c

@@ -33,7 +33,7 @@ int cmdline_get_passwd_input(prompts_t *p)
 int cmdline_process_param(const char *p, char *value,
                           int need_save, Conf *conf)
 {
-    assert(false && "cmdline_process_param should never be called");
+    unreachable("cmdline_process_param should never be called");
 }
 
 /*