/usr/Makefile: Support dports and add error checking

/usr/Makefile: Support dports and add error checking

* Check for git before issuing git command
* Check if directories existing when expected for a target
* Check if directories do not exist when they shouldn't for a target
* Issue error messages when above criteria are not met
* Convert to prinf to make spacing changes easier in future
* Remove options from help screen that are currently invalid
  ("make" by itself now reflects only options that are valid at the time)
* Remove reference to cvs repositories, that's obsolete now
* Add "dports-create" option (full git repository cloned from github)
* Add "dports-create-shallow" option (Depth=1 repository github clone)
* Add "dports-update" option to pull latest from git repository
* Add "dports-download" option (Download tarball from github, extract)
  (Has benefit that dports can be installed without git on system).
* Add "dports-replace" option (When dports was previously installed from
  tarball, use the option to replace it by downloading and extracting
  newer tarball)
* general cleanup