change WIF to "txin_type:old_wif"

[SomberNight]

• new versions always export as txin_type:wif (0x80 first byte)
  • we no longer import and export private keys "as-is". when exporting, everything is re-serialized as txin_type:wif, even minikeys
• new versions allow importing
  • txin_type:wif (only 0x80 first byte allowed)
  • wif -- and this format allows other first bytes; guess txin_type based on them
    • provides backwards compat for our "3.0.x extended WIF"
• internally, Imported Wallet keystores keep storing privkeys using "3.0.x extended WIF"

Related: #3620, #3881