Consolidate do_subset_xts() and _do_subset_xts()

[joshuaulrich]

_do_subset_xts() is used everywhere except for src/na.c. Test that using it in src/na.c doesn't break anything internally (i.e. na.omit.xts() and na.exclude.xts()).

We also need to ensure it doesn't change the output of na_omit_xts(), which is callable externally via R_RegisterCCallable() and xtsAPI.h. That said, neither of the *do_subset_xts() functions are directly callable externally (i.e. via R_RegisterCCallable() or via xtsAPI.h), so we can swap the functionality of one for the other.