Use public domain SHA1 code

Use public domain SHA1 code

I believe that technically I shouldn't have been using the coreutils SHA1 code
in wimlib because the coreutils code is GPLv3 while wimlib is LGPLv2.1+.  So I
replaced it with public domain code.

This only applies to building SHA1 support into wimlib.  I would recommend using
an external libcrypto if possible.