first change: pimpl fully applied to IOStreams file_descriptor, tested on linux/gcc-4.1.2

svn path=/sandbox-branches/birbacher/tmp_iostreams/boost/iostreams/; revision=39109
fixed special constructor definition for Windows platform

svn path=/sandbox-branches/birbacher/tmp_iostreams/libs/iostreams/; revision=39111
extended test suite for error testing in IOStreams, tested on windows/msvc8express

svn path=/sandbox-branches/birbacher/tmp_iostreams/libs/iostreams/; revision=39180
minor modification of test cases in IOStreams error_test.cpp

svn path=/sandbox-branches/birbacher/tmp_iostreams/libs/iostreams/; revision=39182