related to Mantis #3121, make SEXP parsing more robust, and remove some old debug/unused code in get_sexp_main()

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11113 387891d4-d844-0410-90c0-e4c51a9137d3
better method of appending ellipsis (nb: strncpy pads with 0 if string is shorter than len)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11114 387891d4-d844-0410-90c0-e4c51a9137d3
technically, this is better

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11115 387891d4-d844-0410-90c0-e4c51a9137d3