tmpfiles: check relative L? targets beside the link

tmpfiles: check relative L? targets beside the link

Relative symlink targets are resolved from the directory containing the
link. Make the L? existence check use the same rule.

Reproducer:
  tmp=$(mktemp -d /tmp/tmpfiles-link.XXXXXX)
  conf=$(mktemp /tmp/tmpfiles-conf.XXXXXX)
  printf data >"$tmp/target"
  printf 'L? %s/link - - - - target\n' "$tmp" >"$conf"
  systemd-tmpfiles --create --dry-run "$conf"

Before:
  The target was checked root-relative, not beside the link.

Follow-up for b5dc805583bbb019c6bf4c73bf0814a396dc0f12.