The old GetAdditionalGroups* API didn't match the rest of
libcontainer/user, we make functions that take io.Readers and then make
wrappers around them. Otherwise we have to do dodgy stuff when testing
our code.
Fixes: d4ece29c0b ("refactor GetAdditionalGroupsPath")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>