gdb: make inferior::terminal a unique ptr

This changes the inferior::terminal field to be a unique pointer, so its
deallocation is automatically managed.

gdb/ChangeLog:

	* inferior.h (struct inferior) <terminal>: Change type to
	gdb::unique_xmalloc_ptr<char>.
	* inferior.c (inferior::~inferior): Don't free inf->terminal.
	* infcmd.c (set_inferior_io_terminal): Don't free terminal
	field, adjust to unique pointer.
	(get_inferior_io_terminal): Adjust to unique pointer.

Change-Id: Iedb6459b4f9eeae812b0cb9d514b5707d5107cdb