path-lookup: if $HOME can be determined but $XDG_RUNTIME_DIR can't, is it

path-lookup: if $HOME can be determined but $XDG_RUNTIME_DIR can't, is it

So far, if either $HOME or $XDG_RUNTIME_DIR is not set we wouldn't use
either, and fail acquire_config_dirs() and acquire_control_dirs() in
their entireties. With this change, let's make use of the variables we
can acquire, and don't bother with the other.

Specifically this means: in both acquire_config_dirs() and
acquire_control_dirs() handle ENXIO from user_config_dir() and
user_runtime_dir() directly, instead of propagating it up and handling
it in the caller.