pointcache: Set start frame soft limit to 1

Right now most simulations can't handle cache start frame < 1
unfortunately.

There are two main problems:
1. frame 0 is a special case so some simulations will just pop back to
their initial state on frame 0.
2. blender doesn't allow playing the animation starting from negative
start frames, so most simulations only work when baked and not during
regular playback.

Just change soft limit since sart frame < 1 can work in some cases, but
this needs to be fixed properly in the individual simulations if we
really want to allow it.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54629 954f8c5b-7b00-dc11-b283-0030488c597c