In preparation for removing the point cache memory frame storage, set
all point caches to 'unbaked' to enfore rebaking.
Disabled readfile/writefile code for memory point cache. In future all
caches will be assumed to be disk caches, with the option of packing
them into .blend files.
Removed mem_cache access in pointcache.c, i.e. assume that the
PTCACHE_DISK_CACHE flag is always set.
Removed the disk/memory toggle for point cache.
Pass ListBase mem_cache as an explicit argument to the point cache
conversion methods. This will no longer be stored in the PointCache
DNA struct (all caches are disk based now).

ParticleSystem stores this as a separate list now, so it can keep using
it in edit mode and for trails drawing.
Replaced the mem_cache pointer in PointCache by a custom ListBase in
ParticleSystem PTCacheEdit for the particle edit mode. This is probably
broken like hell, but need to get rid of this for a clean point cache
API. Have to come back to particle edit mode later once we have proper
point cache in place.
Removed remaining pointcache->mem_cache access in particles. Instead the
particle system should now use its own local mem_cache list for storing
cache data in memory. The actual PointCache struct does not make this
distinction any more.

Eventually the particle system should have clean methods for reading
cache data into other structures for editing and displaying cache data,
but this can not be cleaned up easily without major refactoring. For now
the important thing is to keep psys away from the PointCache internals
so we can implement a nice API for it without psys interfering.
Removed a few unused variables.
Made the PTCACHE_DISK_CACHE flag deprecated and removed all uses. This
is assumed to be TRUE always, all caches are disk-based now, which
removes a lot of complexity from constant double-checking. Eventually
disk caches can be integrated into .blend files, but this will be a
secondary feature instead of an alternative format for caches, so we
don't have to implement every function in 2 different ways.
Removed deprecated DNA data.

This will break forward compatibility, but this is broken either way,
there's no way we can smoothly convert new point cache data into the old
format without considerable effort. It's just a few settings for cache
behavior anyway, no valuable user work getting lost.