Added tests and fixed Dashtap function 'varname'.

* No longer allows '_' and the empty string as variable names.
Added Dashtap function 'strip_newline'.
Rephrased error messages.
Added Dashtap function 'setread' + its test cases.

* 'setread' is used in the same way as 'seteval' but instead of
  evaluating an expression it reads its input from a file or here
  document.
Renamed Dashtap test file.

* 'xt/dashtap-evalset.t' -> 'xt/dashtap-seteval.t'.
Refactored Dashtap function 'execute'.