Moved the new point cache settings DNA data into the existing PointCache
struct. Adding new DNA data alongside the existing structs leads to
a lot of name conflicts, better try to stay within existing structs and
gradually extend/replace struct members.
Made all point cache DNA members relating to cache lists DNA_DEPRECATED.
This includes the ListBase ptcaches members in all the users as well as
the next/prev pointers in the PointCache struct.
Comment on smoke caches is deprecated.
Adapted bpath point cache traversal macro to single PointCache pointers.
Revert "Comment on smoke caches is deprecated."
Thought this was referring to the ptcaches ListBase, but actually means
the point_cache array, so i still important.

This reverts commit 8f48294bd00b61bae93f375dc0a730cac7fb594e.
Point cache index 1 in smoke domain is not used.
Removed the ptcache ListBase linking in readfile. Only the single
point_cache pointers are used now, which conveniently are already the
active point caches, so we can just ignore the lists (setting them to
the same pointer though to avoid potential problems with forward
compatibility).
Changed point cache writefile function to only write the point cache
struct, ptcaches lists are deprecated.
Removed ListBase code for multiple point caches from blenkernel and RNA.
Removed ptcache list code in readfile.c. This was intended to provide
some forward compatibility, but it's just not feasible to support this
nicely, so better accept breaking forward compat and don't touch this
DNA any more (keep code simple).