Eliminate user 65535

[asomers]

Only two unprivileged users are needed: 65534 and 65533 suffice.

chmod/00.t This test only needs one unprivileged user.

{mkfifo,open,mkdir,mknod}/00.t These tests all related to the group ID of a newly created file. They need two unprivileged users and three groups.

[asomers]

cc @musikid

[asomers]

musikid's GSoC project precipitated this change. For the rewrite we tried to reduce the number of extra users to the minimum, and put them into a config file instead of hard-coding them into each test. The rewrite is nearly done, so I don't intend to keep developing the C/sh version. But I opened this and other PRs as a kind of diff reduction before merging the rewrite. BTW, the new version is at https://github.com/musikid/pjdfstest/tree/master/rust .