secrets: fix replacemend random seed generator (security issue).

secrets: fix replacemend random seed generator (security issue).

This is a regression introduced by the change to dbwrap.
The replacement dbwrap_change_int32_atomic() does not
correctly mimic the behaviour of tdb_change_int32_atomic():
The intended behaviour is to use *oldval  as an initial
value when the entry does not yet exist in the db and to
return the old value in *oldval.

The effect was that:
1. get_rand_seed() always returns sys_getpid() in *new_seed
   instead of the incremented seed from the secrets.tdb.
2. the seed stored in the tdb is always starting at 0 instead
   of sys_getpid() + 1 and incremented in subsequent calls.

In principle this is a security issue, but i think the danger is
low, since this is only used as a fallback when there is no useable
/dev/urandom, and this is at most called on startup or via
reinit_after_fork.

Michael