ovsdb-idlc: Constify 'char **'.

ovsdb-idlc: Constify 'char **'.

Update the logic used in constify() to add const to a 'char **' while
still excluding all other cases of more than one level of indirection.

This results in adding const to a parameter of a generated setter
function where we're generally passing in array of constant strings.
As a result, this patch includes the other necessary fixes to the code
base to reflect the const addition.

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>