Fix: enable Piq-speicfic checks back as well as record-level .piq-positional

This code was accidentally disabled by using a wrong linking order which led to
excluding the piq.ml module's functionality.
More robust handling of "convert --piq-frameless-input"

This option is not applicable to scalar types so we need to process them
slightly differently than container types.
piqi server: handle exceptions more accurately

exceptions

  internal error: error while running function "convert": Piqi_convert.EOF
  internal error: error while running function "convert": Piqi_piqirun.Error(-1,
  "missing field: 1")

are now reported as proper piqi-rpc errors: missing_input and invalid_input
respectively