Fixed small typos in comments.
Fixed most of the warnings in dcmjpeg.

Compiled with extensive warning options except -Wtraditional and corrected
most of the problems. Tested with mingw-w64-4.8.1, gcc-4.7.2 (static and shared),
Visual Studio 2013 Express, Visual Studio 2012 64 bit and clang-w64-3.5.0.
Tested compression / decompression with various different settings for different
compilers on appropriate DICOM images ("cabinet" and "car97_cd").
Most of the remaining warnings in dcmjpeg are caused by files included from
dcmimage and will be resolved in a different commit.
Fixed potential bug in dcmjpeg 12 and 16 bit variants.

Applied a fix that was already present in the 8 bit variant to the other
variants of the jpeg library.
Marked SCPPool-Test (dcmnet_scp_pool) as exhaustive.

The current pool-testcase takes about 6 seconds, which is rather long
compared to the other tests. It was therefore flagged as exhaustive test
and is not run unless explicitly enabled.