Add squeeze-lts to codenames.

(This commit was done by Paul Tagliamonte <tag@pault.ag>, but Author has
 been set, since this is entirely made up of a patch from jmm. Thanks,
 jmm.)

 For the Debian LTS efforts there's now the new suite "squeeze-lts".
 For more information:

   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745806
fix secure_sftp.py broken by python changing Popen's bufsize default