Make use of `diagnose_if`

[jvoisin]

Make use of diagnose_if for things like:

• memset called n set to 0, likely indicating that arguments got inverted.
• realpath with path set to NULL, likely indicating that arguments got inverted.
• umask with mode & ~0777.
• printf format strings may only contain the %n format specifier if the format string resides in read-only memory.
  • it's not possible to implement this in a non-cursed way

And add tests for it.