- Fix a few spelling mistakes

- Use sizeof() to pass an array size to a function
- Use a single bit for a variable in the chan_iax2_pvt stuct since that is all
  it needs.
- Add some comments about the iaxs, iaxl, and lastused arrays.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48359 65c4cc65-6c06-0410-ace0-fbb531ad65f3