packages: Raise an exception for invalid 'license' values.

packages: Raise an exception for invalid 'license' values.

This is written in such a way that the type check turns into a no-op at
macro-expansion time for trivial cases:

  > ,optimize (validate-license gpl3+)
  $18 = gpl3+
  > ,optimize (validate-license (list gpl3+ gpl2+))
  $19 = (list gpl3+ gpl2+)

* guix/packages.scm (valid-license-value?, validate-license): New
macros.
(<package>)[license]: Add 'sanitize' option.
(&package-license-error): New error condition type.
* tests/packages.scm ("license type checking"): New test.