Eliminate more warnings.

This eliminates a large set of warnings exposed by the Mozilla build
 system (Use of C++ comments in ISO C90 source, commas at the end of
 enum lists, a couple incomplete initializers, and signed/unsigned
 comparisons).
It also eliminates many (but not all) of the warnings expose by newer
 GCC versions and _FORTIFY_SOURCE (e.g., calling fread and fwrite
 without checking the return values).
There are a few spurious warnings left on my system:

../vp8/encoder/encodemb.c:274:9: warning: 'sz' may be used
 uninitialized in this function
gcc seems to be unable to figure out that the value shortcut doesn't
 change between the two if blocks that test it here.

../vp8/encoder/onyx_if.c:5314:5: warning: comparison of unsigned
 expression >= 0 is always true
../vp8/encoder/onyx_if.c:5319:5: warning: comparison of unsigned
 expression >= 0 is always true
This is true, so far as it goes, but it's comparing against an enum, and the C
 standard does not mandate that enums be unsigned, so the checks can't be
 removed.

Change-Id: Iaf689ae3e3d0ddc5ade00faa474debe73b8d3395