Patch: Elimination of the SLIST_FREIGHT define, use slist_tpl always

[neroden]

This eliminates the vector_tpl implementation of freight lists.  This is basically just a use of 'unifdef', but it provides me with a nice patch as a test case for using git.

OK, let's see what the git way of doing this is:

James, please pull from the repo git@github.com:neroden/simutrans.git on the branch "jp-devel".  (For your devel branch.)

Tell me if this works.

Followup: the corresponding patch for your master branch is on the branch "jp-master".  (There's a silly merge conflict with a meaningless
punctuation fix which I made in one of my earlier patches, hence the difference.)

Followup: I guess it works, excellent!

[jamespetts]

Neroden,

thank you very much indeed for this patch, and apologies for not having responded sooner. As you correctly surmised by its inclusion in the latest push to the -devel branch, it does indeed work. Thank you again for your efforts - they are much appreciated.