crypto/x509: unwrap Subject Key Identifier
crypto/x509: unwrap Subject Key Identifier

RFC 5280, 4.2.1.2 says:
  SubjectKeyIdentifier ::= KeyIdentifier
  KeyIdentifier ::= OCTET STRING

Previously, we were failing to unwrap the second level of OCTET STRING
encoding.

Fixes issue 993.

R=rsc
CC=golang-dev
http://codereview.appspot.com/1917044

bryanxu/go-zh,mhennings/marcohennings-go,d0f/go-zh,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,d0f/go-zh,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,npe9/noahevans-go9,npe9/noahevans-go9,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,d0f/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,npe9/noahevans-go9,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,npe9/noahevans-go9,webfd/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,d0f/go-zh,Triskite/willstone-goclone,scirelli/scirelli-go,webfd/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,webfd/go-zh,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,npe9/noahevans-go9,scirelli/scirelli-go,Triskite/willstone-goclone,rdp/rogerpack2005-golang,npe9/noahevans-go9,scirelli/scirelli-go