sync functions for freeing dock instances

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10059 387891d4-d844-0410-90c0-e4c51a9137d3
turn p_object into a class
ensure that p_object dock instances are cleared between missions, not just when the game exits
handle Player_start_pobject in a saner way
the p_object Reset method is nice, but it turns out we don't need it

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10060 387891d4-d844-0410-90c0-e4c51a9137d3