Fix misleading function name

"usage()" name will mean one of them:

  * It returns "usage string".
  * It shows "usage".
  * ...

But "usage()" will not check arguments because "usage" term doesn't mean
"check" nor "validate".
Merge branch 'fix-misleading-function-name' of https://github.com/kou/BLThunder1991-sezemi-2014-readable-code-2 into kou-fix-misleading-function-name

Conflicts:
	recipe.c
Merge branch 'kou-fix-misleading-function-name'
Merge branch 'master' of github.com:BLThunder1991/BLThunder1991-sezemi-2014-readable-code-2