+ Rename `SMTK_ENABLE_REMUS` to `SMTK_ENABLE_REMUS_SUPPORT` to
  match other options.
+ Add `SMTK_ENABLE_REMOTE_SESSION` rather than just using Remus
  support to trigger build of remote session (because Remus now
  has a non-session extension library for the mesh operator).
+ Mark some settings as advanced. Now all the (non-advanced)
  SMTK settings should fit on 26 lines (23 on non-Mac-OS
  platforms).