`usize` is made exactly for this (e.g.: the size of a buffer), and
signatures are a bit more idiomatic.

These changes also try to cast the u32 to a usize as soon as we read it,
and will panic if there is an issue (whereas using `as usize` may result
in silent conversion errors on uncommon architectures.

Rather than try a single path, try all supported paths, in the preferred
order. This avoids breaking compatibility with previous KeePassXC
versions.

See: keepassxreboot/keepassxc#8018
Fixes: varjolintu#8
Supercedes: varjolintu#7