arch/um: make it work with defconfig and x86_64

arch/um/defconfig only lists one default configuration, and that applies
only to the i386 architecture.  Replace it with two files:

  i386_defconfig and x86_64_defconfig

The build scripts now require two updates:

1. um's Kconfig (arch/x86/um/Kconfig) should specify an ARCH_DEFCONFIG
   section explicitly pointing to these scripts if the required
   variables are set.  Take care to remove the DEFCONFIG_LIST section
   defined in the included file arch/um/Kconfig.common.

2. um's Makefile (arch/um/Makefile) should set KBUILD_DEFCONFIG properly
   for the top-level Makefile to pick up.  Copy the logic in
   arch/x86/Makefile to properly pick the defconfig file depending on
   the actual architecture; except we're working with $SUBARCH here,
   instead of $ARCH.

Now, you can do:

  $ ARCH=um make defconfig
  $ ARCH=um make

and successfully build User-Mode Linux on an x86_64 box in default
configuration.

Cc: Richard Weinberger <richard@nod.at>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
um: change defconfig to stop spawning xterm

The default configuration dictates that um Linux will launch xterms
connected to various tty devices on the host.  It makes more sense to
connect to /dev/pts/N, so that the host can:

  $ screen /dev/pts/N

or

  $ minicom /dev/pts/N

to start talking to the guest session.  Note that
con0 (CONFIG_CON_ZERO_CHAN) still defaults to "fd:0,fd:1", printing boot
messages on the same terminal.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>