Eliminate use of _impure_ptr outside Cygwin. * bsd_helper.cc: Include errno.h instead of cygerrno.h. * bsd_mutex.cc : Ditto. * client.cc: Ditto. * cygserver.cc: Ditto. * process.cc: Don't build functions inside Cygwin. Don't include cygerrno.h. Don't set errno. * transport_pipes.cc (SET_ERRNO): New define. Use througout. * transport_sockets.cc (SET_ERRNO): Ditto. (GET_ERRNO): Ditto.