Convert inflow to type-safe registry API

Convert inflow to type-safe registry API

This changes inflow.c to use the type-safe registry API.  This fixes a
latent bug in swap_terminal_info, which previously said:

  terminal_info *info_a
    = (terminal_info *) inferior_data (a, inflow_inferior_data);
  terminal_info *info_b
    = (terminal_info *) inferior_data (a, inflow_inferior_data);

... both of which examine 'a'.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* inflow.c (struct terminal_info): Add destructor and
	initializers.
	(inflow_inferior_data): Change type.
	(~terminal_info): Rename from inflow_inferior_data_cleanup.
	(get_inflow_inferior_data, inflow_inferior_exit)
	(swap_terminal_info, _initialize_inflow): Update.