connection: add createDefaultBusConnection()

This internally calls sd_bus_open(), which automatically selects the
system or session bus connection based on the presence and 
content of a DBUS_STARTER_BUS_TYPE environment variable and
whether the calling process has root privileges.

This option is very helpful when creating services and clients that will use the system bus in production, but connect to a session
for testing.

Additional changes:
* Removed assertions null-checking make_unique() return values.
  make_unique() calls new, and new is expected to throw or abort
  on failure, making the assertions unhelpful.
* Corrected a typo in the ClosesAndUnrefsBusWhenDestructed
  unit test for the system bus (tested the wrong function).

src/Connection.cpp

@@ -44,6 +44,11 @@ Connection::Connection(std::unique_ptr<ISdBus>&& interface, const BusFactory& bu
     assert(iface_ != nullptr);
 }
 
+Connection::Connection(std::unique_ptr<ISdBus>&& interface, default_bus_t)
+    : Connection(std::move(interface), [this](sd_bus** bus){ return iface_->sd_bus_open(bus); })
+{
+}
+
 Connection::Connection(std::unique_ptr<ISdBus>&& interface, system_bus_t)
     : Connection(std::move(interface), [this](sd_bus** bus){ return iface_->sd_bus_open_system(bus); })
 {
@@ -484,10 +489,23 @@ std::unique_ptr<sdbus::IConnection> createConnection(const std::string& name)
     return createSystemBusConnection(name);
 }
 
+std::unique_ptr<sdbus::IConnection> createDefaultBusConnection()
+{
+    auto interface = std::make_unique<sdbus::internal::SdBus>();
+    constexpr sdbus::internal::Connection::default_bus_t default_bus;
+    return std::make_unique<sdbus::internal::Connection>(std::move(interface), default_bus);
+}
+
+std::unique_ptr<sdbus::IConnection> createDefaultBusConnection(const std::string& name)
+{
+    auto conn = createDefaultBusConnection();
+    conn->requestName(name);
+    return conn;
+}
+
 std::unique_ptr<sdbus::IConnection> createSystemBusConnection()
 {
     auto interface = std::make_unique<sdbus::internal::SdBus>();
-    assert(interface != nullptr);
     constexpr sdbus::internal::Connection::system_bus_t system_bus;
     return std::make_unique<sdbus::internal::Connection>(std::move(interface), system_bus);
 }
@@ -502,7 +520,6 @@ std::unique_ptr<sdbus::IConnection> createSystemBusConnection(const std::string&
 std::unique_ptr<sdbus::IConnection> createSessionBusConnection()
 {
     auto interface = std::make_unique<sdbus::internal::SdBus>();
-    assert(interface != nullptr);
     constexpr sdbus::internal::Connection::session_bus_t session_bus;
     return std::make_unique<sdbus::internal::Connection>(std::move(interface), session_bus);
 }
@@ -517,7 +534,6 @@ std::unique_ptr<sdbus::IConnection> createSessionBusConnection(const std::string
 std::unique_ptr<sdbus::IConnection> createRemoteSystemBusConnection(const std::string& host)
 {
     auto interface = std::make_unique<sdbus::internal::SdBus>();
-    assert(interface != nullptr);
     constexpr sdbus::internal::Connection::remote_system_bus_t remote_system_bus;
     return std::make_unique<sdbus::internal::Connection>(std::move(interface), remote_system_bus, host);
 }