crypto: Correct s2k type constants. They are 0,1,3 according to both the RFC: http://tools.ietf.org/html/rfc4880#section-3.7
And according to the gpg implementation: http://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git&a=search&h=refs%2Fheads%2FSTABLE-BRANCH-2-0&st=grep&s=s2kmode

R=agl
CC=evn, golang-dev
https://golang.org/cl/11355046