Returns the sigflag from a NUT-11 P2PK secret.
The NUT-11 P2PK secret.
The sigflag ('SIG_INPUTS' or 'SIG_ALL').
'SIG_INPUTS'
'SIG_ALL'
If secret is not P2PK, or if the sigflag tag contains an unrecognised value.
Returns the sigflag from a NUT-11 P2PK secret.