Improve OpenBSD compatibility

[adp-psych]

This pull request contains various changes that I wanted on an OpenBSD server:

• it skips PKGS-7398 and AUTH-9262, which are impossible to satisfy on OpenBSD;
• it ignores a user named git when checking for unique UIDs because git is a common/convenient username for SSH connections to a Git server;
• it ignores the git, _gitea, and _forgejo accounts when checking login shells because these passwordless accounts need a real shell to work properly;
• it checks homedir ownership via UID rather than username, avoiding duplicates of warnings for duplicate UIDs; and
• it changes the colour for no swap partitions from yellow to white.