checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/libxaw3d/package.py

@@ -13,7 +13,7 @@ class Libxaw3d(AutotoolsPackage):
     homepage = "http://cgit.freedesktop.org/xorg/lib/libXaw3d"
     url      = "https://www.x.org/archive/individual/lib/libXaw3d-1.6.2.tar.gz"
 
-    version('1.6.2', 'e51e00b734853e555ae9b367d213de45')
+    version('1.6.2', sha256='847dab01aeac1448916e3b4edb4425594b3ac2896562d9c7141aa4ac6c898ba9')
 
     depends_on('libx11')
     depends_on('libxt')