optional enhancements:
- RPC use persistent client connection (configurable)
  add support for __enter__ and __exit__ for "with"
- getmntent: distinguish end-of-list from error (when possible)?
- use tirpc even if SUN-RPC is implemented in libc too:
  requires linker options to control symbol resolution
- unit-tests (as far as possible)
- automatically define NDEBUG for debug builds (if possible?)
- generate UNIX man page for dist
