oggpack_read() return value not well defined
If <long> is 32 bit on your platform, the return value -1 can mean
two different things: an out of data error occured, or we just read
32 bits, all 1.
And there's a signed/unsigned issue as well.
I'll convert everything that needs to be unsigned, in ogg as well as
in vorbis.
This will get rid of a lot of compiler warnings on non-x86 and non-gcc
platforms...