Add changes for DOS:

Add changes for DOS:

- Add dos-specific packages, similar to Unix packages.
- Conditionalize the loading of the Unix and DOS -specific packages.
- Add a mechanism to make.scm to avoid loading unconditional files:
  If the binary file runtim.bad is in the current directory when make
  is running, it should contain a list of strings, each the name of a
  file that should be avoided.  Examples: graphics, x11graph, starbase.
- pathnm.scm now uses the OS-NAME-STRING in the microcode
  identification to initialize the default host.  It no longer
  constructs the host types.  These are constructed when the
  system-dependent files invoke add-pathname-host-type!
  Caveat: There is now a funny ordering dependency.
    pathnm must be loaded before the system-dependeng files (unxpth, dospth)
    but it must be initialized _afterwards_!
- pathnm.scm now has symbolic names for three file system types: Unix,
  DOS, and VMS.