Bring repodata_lookup_num in sync with repo_lookup_num

Bring repodata_lookup_num in sync with repo_lookup_num

It used to return a boolean if the num was found and write
the num over a pointer. This was done because repo_lookup_num
needed to iterate over all repodatas and thus needed to know
if the keyname existed or not.

Nowadays we use repo_lookup_repodata_opt to find the correct
repodata, so we do no longer need the boolean return value.
So we can make repodata_lookup_num work like repo_lookup_num.

This is an INCOMPATIBLE API CHANGE but I'm pretty sure nobody
uses repodate_lookup_num, so this should not hurt anybody.

(You can use a call to repodata_lookup_type to check if the
repodata really contains the num in case the "notfound" value
is returned.)