Use 'shell_expand_guest_path' guest capability to expand "~/" in shared folder mounts

This was copied from the official NFS mount capability
Emit event for Upstart configs when shared folders are mounted

This was copied from NFS and Virtualbox implementation.
We try to send a special 'vagrant-mounted' event to upstart so
that services that specially subscribe to it can do a 'reload'

For instance, if nginx to start serving content of a freshly
mounted directory, it needs to be 'pinged'.