Fix issue with ControlMaster sockets pruning.

The prune_ssh_sockets shell function would always delete the
ControlMaster sockets (the open file handlers would have a random suffix
and the check would always show that the socket wasn't open by any
process). Instead add a prefix to the ControlMaster sockets and ignore
them when pruning. Those sockets are always deleted when the process
exists. Also, mark the vim filetype for the ssh config fragments as
sshconfig.