fixup printing methods in devtools/decodemsg such that TLV's can
now be printed as well. here's how you'd use it:
$ ./devtools/decodemsg --tlv opening_tlv 0120001E020202020202020202020202020202020202020202020202020202020202
> WIRE_OPTION_UPFRONT_SHUTDOWN_SCRIPT (size 32):
> shutdown_scriptpubkey=[020202020202020202020202020202020202020202020202020202020202]