portable: keep unit symlinks inside the image

Open unit files with chase_and_openat() rooted at the image instead of
plain openat(). Symlinks are now resolved under the image root and must
end at a regular file.

Reproducer:
  ln -s /tmp/portable-host-unit \
      IMAGE/usr/lib/systemd/system/PREFIX.service
  portablectl inspect --cat IMAGE PREFIX.service

Before:
  inspect followed the absolute symlink on the host and printed host
  file contents as image unit metadata. Image inspection could leak or
  trust files outside the portable image.

Follow-up: 61d0578b07

src/portable/portable.c

@@ -392,9 +392,15 @@ static int extract_now(
                         if (!IN_SET(de->d_type, DT_LNK, DT_REG))
                                 continue;
 
-                        fd = openat(dirfd(d), de->d_name, O_CLOEXEC|O_RDONLY);
+                        fd = chase_and_openat(
+                                        rfd,
+                                        dirfd(d),
+                                        de->d_name,
+                                        CHASE_MUST_BE_REGULAR,
+                                        O_RDONLY|O_CLOEXEC,
+                                        /* ret_path= */ NULL);
                         if (fd < 0) {
-                                log_debug_errno(errno, "Failed to open unit file '%s', ignoring: %m", de->d_name);
+                                log_debug_errno(fd, "Failed to open unit file '%s', ignoring: %m", de->d_name);
                                 continue;
                         }
 

test/units/TEST-29-PORTABLE.directory.sh

@@ -24,6 +24,14 @@ rm -rf /tmp/mismatched-name
 cp -a /tmp/minimal_0 /tmp/mismatched-name
 portablectl inspect /tmp/mismatched-name | grep -F "minimal-app0.service" >/dev/null
 
+rm -rf /tmp/symlink-unit
+cp -a /tmp/minimal_0 /tmp/symlink-unit
+printf '[Service]\nExecStart=/bin/true\n' >/tmp/portable-host-unit
+rm -f /tmp/symlink-unit/usr/lib/systemd/system/minimal-app0.service
+ln -s /tmp/portable-host-unit /tmp/symlink-unit/usr/lib/systemd/system/minimal-app0.service
+(! portablectl inspect --cat /tmp/symlink-unit minimal-app0.service)
+rm -f /tmp/portable-host-unit
+
 portablectl "${ARGS[@]}" attach --copy=symlink --now --runtime /tmp/minimal_0 minimal-app0
 
 systemctl is-active minimal-app0.service