When using siphash24(), the hash value depends on the hashed input
and the key from _get_hash_key(). If the input is static, so is also
the result of siphash24(), albeit the bits are scrabbled more.
Add a nm_hash_static() to get such a static key, but without actually
doing siphash24(). The static key is also xored with a static_seed.
For that, also mangle the first byte of the hash key using siphash24()
itself. That is, because nm_hash_static() only uses the first guint of the
random key. Hence, we want that this first guint has all the entropy
of the entire key. We use siphash24() itself, to mangle all bits
of the 16 byte key into the first guint.