caps: Rename to features to avoid confusion

Git already has a very clear concept of "capabilities" (i.e. the ones that the network protocol supports, such as out of band packets and so on). Calling our own build flag check "capabilities" too makes this confusing, and worse of it all, gives us identifiers like `GIT_CAP_HTTPS` and `GIT_CAP_OFS_DELTA` that are 100% unrelated, and yet have the same prefix.

Let's rename our own build features as "features" because it's a much more appropriate name.