Allow SPDX license expression syntax in the License field.
    
    The Software Package Data Exchange (https://spdx.org/) is a Linux
    Foundation project with overlap with the Fedora license expression
    syntax.  Many projects are beginning to add SPDX-Identifier notation
    to source files and use SPDX short names.  Fedora spec files should
    allow this in the License field as well.
    
    Some limitations:
    
    * Fedora packagers need to use either the Fedora syntax or the SPDX
      syntax; they cannot mix the two.
    
    * SPDX is more expressive with regard to GPL, LGPL, and BSD variants
      which may require packagers to look at the code in detail again.
      For license short names where we have combined all in to one short
      name and SPDX breaks it out, we should cross reference the License
      tag expression with the source if a packager changes to SPDX syntax.
    
    * SPDX offers a list of license exceptions you can put in the
      expression using the WITH keyword[1].  Fedora syntax tends to put
      that as freeform in the License expression.  Where there is an SPDX
      identifier, that should be used.
    
    * If a current Fedora license expression cannot be translated to SPDX,
      it should remain using the Fedora syntax.
    
    * "Redistributable" is not a valid SPDX identifier.
    * "Public Domain" is not a valid SPDX identifier[2].
    
    It would be nice to offer Fedora packagers the option of using SPDX
    syntax or our existing syntax.  The SPDX expressions are easier to
    validate programatically too.
    
    [1] https://spdx.org/licenses/exceptions-index.html
    [2] https://wiki.spdx.org/view/Legal_Team/Decisions/Dealing_with_Public_Domain_within_SPDX_Files