ctld: Convert to C++

[jhb]

Summary

This is the minimal set of changes need to compile as C++ so git can
handle the rename correctly.

[asomers]

A few functions, like conf_set_debug, will now be extern "C" in the header file, but not in their implementation. It might work, but I don't think it's right. Also, conf.c seems to have two purposes. It defines C functions used by parse.y, and it also defines C functions used by ctld.cc. Maybe we should leave conf.c as a C file, but move some its ctld functions into ctld.cc itself? OTOH, if you're going for "minimal set of changes necessary to build as C++, it would be easier just to move those prototypes out of the "extern C" section.

[jhb]

Hmm, my assumption is that the extern "C" for the prototype is sufficient for the compiler to use the "C" symbol for the function when compiling conf.cc, but that permits using C++ constructs in conf.cc. In the future these methods in conf.cc will be invoking methods on C++ objects so it needs to be a conf.cc file, just that the symbols exported from it use C symbol names (not mangled).

Hmm, cppreference.com has an explicit example of this case that it says is fine:

https://en.cppreference.com/w/cpp/language/language_linkage

In particular:

A redeclaration of an entity without a linkage specification inherits the language linkage of the entity and its type (if exists).

Followed by this example:

extern "C" int g();
int g(); // OK, has C language linkage

[asomers]

A redeclaration of an entity without a linkage specification inherits the language linkage of the entity and its type (if exists).

Ahh, good find!