RAUC install handlers execute in a mount namespace which sees the mounted
bundle file. Add HandlerContext.HandlerPID which tracks the process
which was responsible for the request.

This allows sysotad to, receive the request, complete the request using
the file system as the handler process sees it, before responding to the
handler process.

The Service.{Pre,Post}Install methods are extended to take the special
dbus.Sender argument. The service calls org.freedesktop.DBus method
GetConnectionUnixProcessID to find the process corresponding to the sender.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>