guests/bsd: Print a very descriptive error when mounting vbox folders

BSD-based guests do not support VirtualBox shared folders. This is a
common source of confusion in Vagrant. This new error clearly explains
that this is not a bug in Vagrant and provides instructions on how to 
disable them.

lib/vagrant/errors.rb

@@ -784,6 +784,10 @@ module Vagrant
       error_key(:virtualbox_mount_failed)
     end
 
+    class VirtualBoxMountNotSupportedBSD < VagrantError
+      error_key(:virtualbox_mount_not_supported_bsd)
+    end
+
     class VirtualBoxNameExists < VagrantError
       error_key(:virtualbox_name_exists)
     end

plugins/guests/bsd/cap/virtualbox.rb

@@ -0,0 +1,16 @@
+module VagrantPlugins
+  module GuestBSD
+    module Cap
+      class VirtualBox
+        # BSD-based guests do not currently support VirtualBox synced folders.
+        # Instead of raising an error about a missing capability, this defines
+        # the capability and then provides a more detailed error message,
+        # linking to sources on the Internet where the problem is
+        # better-described.
+        def self.mount_virtualbox_shared_folder(machine, name, guestpath, options)
+          raise Vagrant::Errors::VirtualBoxMountNotSupportedBSD
+        end
+      end
+    end
+  end
+end

plugins/guests/bsd/plugin.rb

@@ -26,6 +26,11 @@ module VagrantPlugins
         Cap::NFS
       end
 
+      guest_capability(:bsd, :mount_virtualbox_shared_folder) do
+        require_relative "cap/virtualbox"
+        Cap::VirtualBox
+      end
+
       guest_capability(:bsd, :remove_public_key) do
         require_relative "cap/public_key"
         Cap::PublicKey

templates/locales/en.yml

@@ -1355,6 +1355,25 @@ en:
         The error output from the command was:
 
         %{output}
+      virtualbox_mount_not_supported_bsd: |-
+        Vagrant is not able to mount VirtualBox shared folders on BSD-based
+        guests. BSD-based guests do not support the VirtualBox filesystem at
+        this time.
+
+        To change the type of the default synced folder, specify the type as
+        rsync or nfs:
+
+            config.vm.synced_folder ".", "/vagrant", type: "nfs" # or "rsync"
+
+        Alternatively, if you do not need to mount the default synced folder,
+        you can also disable it entirely:
+
+            config.vm.synced_folder ".", "/vagrant", disabled: true
+
+        You can read more about Vagrant's synced folder types and the various
+        configuration options on the Vagrant website.
+
+        This is not a bug in Vagrant.
       virtualbox_name_exists: |-
         The name of your virtual machine couldn't be set because VirtualBox
         is reporting another VM with that name already exists. Most of the