trap -p: make printing specific sigs work properly (re: 986c2a38)
trap -p: make printing specific sigs work properly (re: 986c2a38)

It now works the same way as printing all traps, including the
quiet conversion of non-modernish traps.

bin/modernish:
- _Msh_POSIXtrap(): If a list of signals was given to -p/--print,
  then no longer die on invalid signal, but warn, and return status
  1 if there are any invalid signals. Store all valid signals into
  a variable.
- _Msh_printSysTrap(): If that variable is set, do nothing if the
  signal doesn't appear in it.

modernish/modernish