Fix a potential problem with reading tokens larger then BUFSIZ

Fix a potential problem with reading tokens larger then BUFSIZ

If io_get() is not able to find the end marker it will go into
an infinite loop. To fix this make it possible to reallocate and
increase the buffer if necessary.

To avoid problems with callers reading into a stack allocated buffer,
change these to no longer use the "zero-copy" strategy. This in turns
cleans the callers up a bit.