sshpeer: more thorough shell quoting

This fixes an issue spotted by Jesse Hertz.

--HG--
branch : stable
extra : rebase_source : d782e89c2d028b2b4eca9a50d2c97bfeda127d98
extra : amend_source : d98609075d5e5e295151c41d2f67b1390bb1558f