CURLOPT_WRITEDATA.3: move version info to AVAILABILITY
curl.h: reverse the enum/define setup for old symbols

We now provide the "real" names in the CINIT() macro setup for CURLOPT_*
symbols, and we provide backwards compatibility defines for the old
symbols as defines instead of vice versa.

This allows us to better use the CINIT() list to check for existing and
current option names.