Due to a bug in unbound, we were passing a string containing a null
character to ub_ctx_resolvconf and ub_ctx_hosts rather than a NULL
pointer. On *nix this wasn't causing headache, but on Windows this was
causing unbound to not correctly load DNS settings from the OS.
Note on the bug: in a Windows-specific code branch in the function
ub_ctx_hosts(), if the hosts file specified was a NULL pointer, a call
to getenv() was stored in a local char* and later freed. This is
incorrect, as we do not own that data, and caused the program to crash.
On Windows, getaddrinfo is part of the Windows API and as such is
__stdcall, not __cdecl, so check_function_exists fails because the
declaration doesn't match the mangling __stdcall has. Instead, use a
header to include the symbol as declared on the system and use
check_symbol_exists instead.
Tested-By: greatwolf on IRC
