put back varargs in writer

compiler/C_CodeGen.jrag

lib/c/labcomm.c

lib/c/labcomm.h

lib/c/labcomm_dynamic_buffer_reader_writer.c

lib/c/labcomm_dynamic_buffer_reader_writer.h

lib/c/labcomm_fd_reader_writer.c

lib/c/labcomm_fd_reader_writer.h

lib/c/labcomm_private.h

lib/c/test/labcomm_mem_writer.c

lib/c/test/labcomm_mem_writer.h