Clean up rootless.h, add optional Xina compatibility

[opa334]

What does this implement/fix? Explain your changes.

This removes the code checking whether THEOS_PACKAGE_INSTALL_PREFIX is define, as this is now always defined.
Additionally it introduces backwards support for xina with the define XINA_SUPPORT that a dev can set inside the Makefile if Xina support is desired. Note that this should only be set for rootful builds.

[leptos-null]

Nice solution to implementing ROOT_PATH_VAR in C.

This still has an issue in a situation such as

char *getName() {
    char *name = malloc(64);
    // ...
    return ROOT_PATH_VAR(name);
}

however I do not believe it is possible to handle a case like this without leaking memory in C.

It occurs to me that VAR variants of these macros should not be used at all (instead, only the literal parts of paths need to be adjusted), however I think that's a separate discussion.

[opa334]

Yeah I mean you can't really expect the VAR macros to free anything, you always need to free stuff yourself. While yeah I do agree these macros don't have a big use case, they should still exist for completion.

[opa334]

Shit I forgot to address this

the char arrays need to be changed to alloca because otherwise the compiler sometimes overwrites them.

So char outPath[PATH_MAX]; needs to be changed to char *outPath = alloca(PATH_MAX)

[leptos-null]

I see. Per my previous comments though, I think the _VAR variants should be removed. I think it's overall a lot of trouble (for the C string, dealing with stack versus heap; and for the Obj-C string, dealing with ARC or not), and for the very few times when this would actually be used, I feel that it's best for the author to write the code themselves using the THEOS_PACKAGE_INSTALL_PREFIX macro. I'm planning to open a PR to do this, so I'm not concerned presently.