encoding/asn1: handle UTCTime before the year 2000
encoding/asn1: handle UTCTime before the year 2000

UTCTime only has a two digit date field and year values from 50 should
be 1950, not 2050.

R=golang-dev, r, rsc
CC=golang-dev
http://codereview.appspot.com/5729063

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