Cast a few more strings to char *, so that we can compile cleanly against

FreeTDS 0.60.  Update the docs to reflect that we can now compile and run
against all modern releases of FreeTDS (0.60 through 0.82)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@126513 65c4cc65-6c06-0410-ace0-fbb531ad65f3