uml: defconfig tweaks

Tweak the UML defconfig -
      we probably don't need 256 old-style ptys - this slows down udev
noticably
      enable hostfs
      disable slab debugging - another noticable performance hit

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>