Add a GitHub workflow to check for whitespace issues.

If it fails you can check the 'git log --check' output of the workflow
to see what the issue is. E.g. (Excess/unwanted whitespace will be
highlighted in red).

  --- afe5753fa changes made in upstream
  src/http/ngx_http_upstream.c:415: trailing whitespace.
  +
  src/http/ngx_http_upstream.c:417: trailing whitespace.
  +      ngx_http_upstream_backend_ssl_protocol, 0,
  src/http/ngx_http_upstream.c:421: trailing whitespace.
  +      ngx_http_upstream_backend_ssl_cipher, 0,

We make use of this in Unit... specific exceptions can be handled via
gitattributes(5).