Changes necessary and proper to get mgear to build rsync:

Changes necessary and proper to get mgear to build rsync:
- $(mg-define-rule) now takes the name of the variable holding the command
  instead of the command itself.  This generally saves the user a $(value).
- I changed my mind again: $@ in a command is translated to the temp file rather
  than the eventual target.
- Handle commas in commands better.
- Enforce that all files are secondary.