for now:
These may need library (xex.c) changes:
xexcat: handle run/init addresses embedded in a segment longer than
  2 bytes.
xexcat: handle or at least detect DOS 1.0 executables.
xexcat: warn if an an init address occurs before the code it
  references. or at least if it occurs before *any* code!


later:
- add cassio?
- file magic? (finish it, see if file upstream wants it)
- new tool: a8grep. knows about xex files (but works on others), can
  search for screen codes or inverse video, etc. also a8strings.