Bugs

- Added an A3::is_attr type override for 'recur', which needs to be
  treated as an un-expanded 'string' type, most, but not all of the
  time.
- Documented the above in ColRecur.cpp.
- Modified Command.cpp to recognize both type 'duration' and name 'recur'.
- Removed Command::next_mod_group argument coalescing for date types,
  which was not used anyway.
- Improved the error message for unrecognized durations, which previously
  included the word 'date'.
- Modified unit tests to accomodate the above error message change.
- Added bug.972.t unit tests, which fail, because it isn't fixed yet.
- Made A3::dump const so it can be used anywhere.

test/bug.886.t

@@ -43,7 +43,7 @@ my $output = qx{../src/task rc:bug.rc add one due:sund 2>&1};
 like ($output, qr/Created task 1\./, 'sund --> valid date');
 
 $output = qx{../src/task rc:bug.rc add two due:donkey 2>&1 >/dev/null};
-like ($output, qr/was not recognized\./, 'donkey --> invalid date');
+like ($output, qr/was not recognized/, 'donkey --> invalid date');
 
 # Cleanup.
 unlink qw(pending.data completed.data undo.data backlog.data synch.key bug.rc);