treewide: fix assorted issues revealed by the meta checker

Turns out a couple of the licenses were wrong, as well as being strings.

pkgs/development/libraries/libxml2/default.nix

···
   meta = {
     homepage = http://xmlsoft.org/;
     description = "An XML parsing library for C";
-    license = "bsd";
+    license = lib.licenses.mit;
     platforms = lib.platforms.unix;
     maintainers = [ lib.maintainers.eelco ];
   };

pkgs/development/libraries/libxslt/default.nix

···
   meta = with stdenv.lib; {
     homepage = http://xmlsoft.org/XSLT/;
     description = "A C library and tools to do XSL transformations";
-    license = "bsd";
+    license = licenses.mit;
     platforms = platforms.unix;
     maintainers = [ maintainers.eelco ];
   };

pkgs/development/tools/misc/binutils/default.nix

···
 
     /* Give binutils a lower priority than gcc-wrapper to prevent a
        collision due to the ld/as wrappers/symlinks in the latter. */
-    priority = "10";
+    priority = 10;
   };
 }

pkgs/stdenv/generic/default.nix

···
 
           '' + ((builtins.getAttr reason remediation) attrs));
 
-      metaTypes = with lib.types; {
+      metaTypes = with lib.types; rec {
         # These keys are documented
         description = str;
         longDescription = str;
         branch = str;
         homepage = str;
         downloadPage = str;
-        license = either (listOf lib.types.attrs) lib.types.attrs;
+        license = either (listOf lib.types.attrs) (either lib.types.attrs str);
         maintainers = listOf str;
         priority = int;
         platforms = listOf str;
···
         version = str;
         updateWalker = bool;
         executables = listOf str;
+        outputsToInstall = listOf str;
+        position = str;
+        repositories = attrsOf str;
+        isBuildPythonPackage = platforms;
       };
 
       checkMetaAttr = k: v: