Remove unused vsnprintf.c test function
vsnprintf.c: make code compatible with ansi2knr tool
conf-ssl.h: make code compatible with pre-ANSI C compilers
Only use unsetenv() when it is available

AIX 4.3 dosn't support it, for example.
configure: clean up function definitions
doc/Platforms.txt: add OpenBSD 5.4 on HPPA
doc/Platforms.txt: add DragonflyBSD 3.4 on x86_64

Thanks to Götz Hoffart <goetz@hoffart.de>!
doc/Platforms.txt: Add and update more systems

- add i386/unknown/openbsd3.5, gcc 2.95.3
- add x86_64/unknown/linux-gnu, Sun C 5.12
- update mipsel/unknown/linux-gnu, gcc 4.4.5

Thanks to Götz Hoffart <goetz@hoffart.de>!
Fix spelling