Add magic-fields to long-lasting data structures

[KristianLyng]

This is a simple memory-corruption safeguard that's relatively common:

foo.h:

struct CompFoo {
#define FOO_MAGIC 0x0112112
int magic;
char *blatti;
...
};

foo.c:

struct CompFoo *f = malloc....
f->magic = FOO_MAGIC;
...

other.c:

int otherFunc(CompFoo *f, char bah) {
assert(f->magic == FOO_MAGIC);
...
}

This is a very simple way to catch some basic corruption issues and it's virtually free.