Move client library status codes into separate header file.  Add pure C
implementation of code for creating input datagrams (for client library).

	modified:   src/bruce/client/bruce_client.h
	new file:   src/bruce/client/status_codes.h
	new file:   src/bruce/input_dg/any_partition/v0/v0_write_msg.c
	new file:   src/bruce/input_dg/any_partition/v0/v0_write_msg.h
	new file:   src/bruce/input_dg/partition_key/v0/v0_write_msg.c
	new file:   src/bruce/input_dg/partition_key/v0/v0_write_msg.h