Werner Lemberg

These two commits make sds compile with a C++ compiler. There are still warnings about `long long` and `buf[]`. However, I think that most compilers support those C99 features, only...

Doing so enormously simplifies error handling; you no longer have to check for memory allocation errors in a chain of sds function calls, only after the last call.

The idea is that `sds.c` gets compiled with a C99 compliant compiler, and a C++ program then uses it. To do so, it needs access to `sds.h`, and this commit...

I think having `extern "C"` is common for C libraries...

Improved patch for issue #11.

### Version 3.1.9a ### Details The commands `\pgfsetarrowoptions` and `\pgfgetarrowoptions` are both undocumented. Note that the former appears twice in the manual but lacks an explanation.

### Version 3.1.9a ### Details I think it should be documented in the manual that the concept of 'nested nodes' doesn't work well in general, and that almost always there...