merge xpg4 and legacy id

[Garrett D'Amore]

The semantic difference between id and xpg4/id is quite tiny.

1. xpg4 id behave almost like id when -a is supplied, except that it does not show the primary group in the groups list.

2. with -a, the group is listed in groups list if it is a supplementary group and also the primary group. (Meaning if it shows up in both places, it is listed there.)

The correct answer is to have /usr/bin/id adopt xpg4 id's behavior, and to make -a determine whether the primary group is suppressed or not. Easy, right?

This is unlikely to break anything.

[Garrett D'Amore]

fixes #58 merge xpg4 and legacy id

→ <<cset be9e9922fa8d>>