Add some helper macros to make #ifdef's in sysinstall easier to read

Add some helper macros to make #ifdef's in sysinstall easier to read
and more maintainable.
- WITH_SYSCONS: defined on all arch's that support syscons (currently i386,
  alpha, and ia64)
- WITH_MICE: defined on all arch's that support moused(8) (currently i386,
  alpha, and ia64)
- WITH_SLICES: defined on all arch's that use disk slices (currently i386
  and ia64)
- WITH_LINUX: defined on all arch's that support Linux binary compat
  (currently i386 and alpha)

Approved by:	re