Introduce a NM_HASH_INIT() function. It makes the places
where we initialize a hash with a certain seed visually clear.

Also, move them from "shared/nm-utils/nm-shared-utils.h" to
"shared/nm-utils/nm-macros-internal.h". We might want to
have NM_HASH_INIT() non-inline (hence, define it in the
source file).