If a `name` is specified as an option, it will be used as the id instead
of inferring it from the `guestpath`. If `guestpath` is nil, the `name`
needs to be specified so the folder can be mounted with a name.
This also fixes the synced folder code to allow `guestpath` to be nil.
It was allowed in a previous version for the purpose of preventing a
directory from being auto mounted (#936), but seems to have become an
error at some point after that.
An example of modifying the default /vagrant folder so it doesn't
auto-mount anymore:
config.vm.synced_folder ".", nil, name: "/vagrant"
An example of sharing another folder, but not auto-mounting it:
config.vm.synced_folder ".", nil, name: "foobar"
Fixes#6835.
c3438ff8f6