Requirements:

• trap (without options or operands) should show signals ignored by inheritance in exec
• trap -p (without operands) should show all signals

Todo:

• Update yash_builtin::trap documentation
• Add yash_env::trap::SignalSystem::get_disposition
• Make yash_env::trap::Setting public (re-exported from yash_env::trap::state::Setting)
• Add yash_env::trap::TrapSet::peek_state that returns Result<&Setting, Errno>
  • Obtains the underlying signal disposition if necessary
  • Has a parameter that allows peeking the parent setting if the current is default
• Update yash_env::trap::TrapSet::get_state documentation to mention the peek_state method
• Make yash_builtin::trap::display_traps show all non-default settings, including InitiallyIgnored
• Add a parameter to yash_builtin::trap::display_traps to select which conditions to show
• Add support for the -p option without operands
• Add support for the -p option with operands
• Elaborate yash_env::trap::TrapSet::iter documentation: it does not yield initially ignored conditions