* libc/include/sys/reent.h: Add include of <sys/_types.h>.
No longer include time.h. Add struct __tm to use for
_localtime_buf in the reentrant structure. Add a
_NULL definition to use in initializing the reentrant struct.
* libc/include/sys/config.h: For CYGWIN32 and RTEMS, change
the _READ_WRITE_RETURN_TYPE to _ssize_t which is found in
<sys/_types.h>.
* libc/include/sys/unistd.h: Include <sys/_types.h>.
* libc/time/lcltime.c (localtime): Cast the reentrant struct
_localtime_buf to be struct tm *.
