Consider not depending on strtoull_l and strtoll_l as much

Related to #2552

Quoting myself:

I think removing the dependency on strtoull_l and strtoll_l for most of the USE_XLOCALE code is reasonable (and can send a patch to this effect)

In a more extreme direction, I don't understand what advantages the _l functions bring, and would potentially suggest switching directly to strtoull and strtoll (or using them as fallback for the _l versions). glibc does seem to do some magic with thousand separators, but that's not even implemented by the fallback that glib itself uses, g_parse_long_long.