Set USER and LOGNAME environment variables when dropping privileges

When a service is configured to run as a non-root user (@user), finit
correctly drops privileges via setuid() and sets HOME and PATH, but
does not set the USER and LOGNAME environment variables. They remain
set to "root" from boot time.

This causes problems for software that determines its identity from
the environment rather than getuid(). For example, rootless Podman
checks os.Getenv("USER") first when looking up subordinate UID/GID
ranges in /etc/subuid and /etc/subgid.

With USER=root but UID=1000, Podman looks up root's subuid entry
instead of the actual user's, causing applications like newuidmap
to fail. Setting USER and LOGNAME to match the actual user identity
follows POSIX conventions and matches the behavior of su, sudo, and
login.

Author: aanderse

src/service.c

@@ -627,8 +627,11 @@ static pid_t service_fork(svc_t *svc)
 			set_uid(uid, svc);
 
 			/* Set default path for regular users */
-			if (uid > 0)
+			if (uid > 0) {
 				setenv("PATH", _PATH_DEFPATH, 1);
+				setenv("USER", svc->username, 1);
+				setenv("LOGNAME", svc->username, 1);
+			}
 			if (home) {
 				setenv("HOME", home, 1);
 				if (chdir(home)) {