ATR-24906: Reduce stringency of checks on explicit L1 seed thresholds in chain name

As discussed in ATR-24906, we follow the convention that only HLT legs that need the special thresholds will include them. For FS legs or legacy-seeded legs, they are not needed and not allowed.

Other consistency checks are included to prevent poorly formed chain names and to keep the checks simple (in particular avoidance of 'PROBE').