Remove unused function GetCWD
- No longer necessary to define _POSIX_ for MSVC builds
- Remove some cruft from port.h
- Remove unused direct.h/unistd.h includes
- Clean up includes in iwyu_path_util.h
- No functionality change intended
- Tested on Win32/MSVC + Linux/GCC

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@459 812d1f90-f944-d88a-411a-122bdc93e1ea