(cygwin_internal): Drop CW_SYNC_WINCWD case.
* globals.cc (ro_u_pipedir): New R/O unicode string.
* ntdll.h (RtlSetCurrentDirectory_U): Declare.
* path.cc (cwdstuff::set): Improve comments. Drop setting Win32 CWD to
\\?\PIPE\ on init. Keep Win32 CWD in sync, if possible. Set to
\\?\PIPE\ otherwise.
* include/cygwin/version.h (CYGWIN_VERSION_API_MINOR): Revert to 230.
* include/sys/cygwin.h (cygwin_getinfo_types): Remove CW_SYNC_WINCWD.