The service_already_exists exception type specified in the TS doesn't
have any constructors defined. Since its base class isn't default
constructible, that means has no usable constructors. This may be a
defect in the TS.

This patch fixes it by adding a default constructor, but making it
private. The make_service function is declared as a friend to be able to
call that private constructor.

	PR libstdc++/94199
	* include/experimental/executor (service_already_exists): Add default
	constructor. Declare make_service to be a friend.
	* testsuite/experimental/net/execution_context/make_service.cc: New
	test.