VPP-189 Tweak hash_foreach_pair to avoid static warning

VPP-189 Tweak hash_foreach_pair to avoid static warning

Coverity doesn't like when an incrementing pointer is taken from
the address of a singleton; it cries that this is a potential
buffer overrun.

Since the cases of this in hash_foreach_pair are based on items
from a simple union used just to have different types point to
the same location it's a simple matter of using the pointer to
that location directly. Since we then aren't using the members
of the union, we can change that to an opaque pointer (void *).

This accounts for >60 issues in Coverity.

Whilst here, convert some useful existing comments into a docblock.

Change-Id: I114183ab7d7948d4a6a703451417f79fa37634eb
Signed-off-by: Chris Luke <chrisy@flirble.org>