ssh-config: disallow adding keys to agent

If a user has set AddKeysToAgent=yes in their ssh config then the
temporary SSH key that vagrant creates could get added to their agent
each time an SSHFS mount is performed for a different machine. Let's
disable that.

Fixes #88

lib/vagrant-sshfs/cap/guest/linux/sshfs_forward_mount.rb

@@ -81,6 +81,7 @@ def self.sshfs_forward_mount_folder(machine, opts)
           # Add in some ssh options that are common to both mount methods
           opts[:ssh_opts] = ' -o StrictHostKeyChecking=no '# prevent yes/no question
           opts[:ssh_opts]+= ' -o ServerAliveInterval=30 '  # send keepalives
+          opts[:ssh_opts]+= ' -o AddKeysToAgent=no' # don't add keys to Agent #88
 
           # Do a normal mount only if the user provided host information
           if opts.has_key?(:ssh_host) and opts[:ssh_host]

lib/vagrant-sshfs/cap/host/darwin/sshfs_reverse_mount.rb

@@ -66,6 +66,7 @@ def self.sshfs_mount(machine, opts)
           # Add in some ssh options that are common to both mount methods
           opts[:ssh_opts] = ' -o StrictHostKeyChecking=no '# prevent yes/no question
           opts[:ssh_opts]+= ' -o ServerAliveInterval=30 '  # send keepalives
+          opts[:ssh_opts]+= ' -o AddKeysToAgent=no' # don't add keys to Agent #88
 
           # SSH connection options
           # Note the backslash escapes for IdentityFile - handles spaces in key path

lib/vagrant-sshfs/cap/host/linux/sshfs_reverse_mount.rb

@@ -65,6 +65,7 @@ def self.sshfs_mount(machine, opts)
           # Add in some ssh options that are common to both mount methods
           opts[:ssh_opts] = ' -o StrictHostKeyChecking=no '# prevent yes/no question
           opts[:ssh_opts]+= ' -o ServerAliveInterval=30 '  # send keepalives
+          opts[:ssh_opts]+= ' -o AddKeysToAgent=no' # don't add keys to Agent #88
 
           # SSH connection options
           ssh_opts = opts[:ssh_opts]