Add infastructure for global transaction set string pool

Add infastructure for global transaction set string pool

- Add a pool pointer to to ts members struct and a getter function
- Grab the global pool for rpmte dependency- and file info creation,
  if its NULL then the sets will use private pools of their own.
- Add the (currently) required magic voodoo rain-dance to freeze and
  unfreeze the pool as necessary wrt new element additions: for
  current rpmal and fingerprinting to work, the string pointers
  must be immovable.
- This is infrastructure only: nothing creates the global pool yet,
  so everything is still using private pools.