src: allow use of base integer types as set keys in concatenations

src: allow use of base integer types as set keys in concatenations

"typeof ip saddr . ipsec in reqid" won't work because reqid uses
integer type, i.e. dtype->size is 0.

With "typeof", the size can be derived from the expression length,
via set->key.

This computes the concat length based either on dtype->size or
expression length.

It also updates concat evaluation to permit a zero datatype size
if the subkey expression has nonzero length (i.e., typeof was used).

Signed-off-by: Florian Westphal <fw@strlen.de>