Switch to an unsigned and larger integer type for loaded password hash and salt counts

It may soon be not unreasonable to load more than 2 billion password hashes at once (albeit not in a 32-bit build).

For starters, we should change these in struct db_main:

/* Number of salts, passwords and guesses */
        int salt_count, password_count, guess_count;

and the corresponding uses of %d.

Then there's more, such as this in struct db_salt:

/* Number of passwords with this salt */
        int count;

...changing which to a larger type may unfortunately increase memory usage in case of many single-hash salts.

Start by locating struct db_main and struct db_salt, then trace every use of salt_count, password_count, guess_count, and count, including the corresponding %d format strings. The work is done when these count paths consistently support the intended larger unsigned range and the memory impact of per-salt counts has been assessed.