util: Simplify hash implementation

util: Simplify hash implementation

So far first entries for each hash key are stored directly in the hash
table while other entries mapped to the same key are linked through
pointers. As a result of that, the code is cluttered with special
handling for the first items.

This patch makes all entries (even the first ones) linked through
pointers, which significantly simplifies the code and makes it more
maintainable.