- perhaps change rv=SA_ERR_SYS+errno=ETIMEDOUT to rv=SA_ERR_TIMEOUT;
- perhaps move 1024-buffer of writef into sa_t and make adjustable
- support registry sub-library for system call override
- full test suite for whole API
- support for writev(2)
- sa_buffer(sa, <which>, size)
  SA_BUFFER_UREAD
  SA_BUFFER_UWRITE
  SA_BUFFER_KREAD
  SA_BUFFER_KWRITE
  SA_BUFFER_WRITEF