selinux: avoid implicit conversions in policydb code

Use the identical type for local variables, e.g. loop counters.

Declare members of struct policydb_compat_info unsigned to consistently
use unsigned iterators.  They hold read-only non-negative numbers in the
global variable policydb_compat.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>