nn_symbol queries the names and values of nanomsg symbols.

/*  Returns the symbol name (e.g. "NN_REQ") and value at a specified index.   */
/*  If the index is out-of-range, returns NULL and sets errno to EINVAL       */
/*  General usage is to start at i=0 and iterate until NULL is returned.      */
NN_EXPORT const char *nn_symbol (int i, int* value);

With the addition of nn_symbol to dynamically query nanomsg symbols (constants),
the nn_version function is obsolete.  C programs can get the version information
directly from the #defines and other languages can extract it from the various
NN_VERSION_* symbols.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>