Add "share_private" argument to copy constructor

[rmorehead]

The smart pointer C++ reference counting utilized by the DBus::Private
_pvt pointer isn't thread safe, so sharing/copying DBus:Message
instances across threads leads to crashes/DBus C library asserts as
Private pointer objects are released prematurely.

Rather than trying to make the reference counting itself thread-safe,
and slowing down single threaded usage, a share_private optional
argument is added the copy constructor. The share_private argument
defaults to true, which preserves the existing behavior of sharing the
Private* and associated reference counting.

If the share_private copy constructor argument is instead given as
false, a new instance is created with a new Private _pvt pointer and
unique reference count, which is logically "divorced" from the
original except that is refers to the same C DBus library pointer.

Note that the C DBus library handles its reference counting in
a thread safe manner.

This new object can then be utilized by another thread without
affecting the reference counting of the original.