* sysdeps/mach/hurd/read.c (__libc_read): Just call __pread with
OFFSET argument of (off_t) -1.
* sysdeps/mach/hurd/write.c (__libc_write): Just call __pwrite with
OFFSET argument of (off_t) -1.
* sysdeps/mach/hurd/pwrite.c: New file.
* sysdeps/mach/hurd/pread.c: New file.
* hurd/fd-read.c (_hurd_fd_read): Take new parameter OFFSET and
pass it in the RPC instead of always -1.
* hurd/fd-write.c (_hurd_fd_write): Likewise.
* hurd/hurd/fd.h: Update decls.
