Change input_handler to take a unique_xmalloc_ptr

Change input_handler to take a unique_xmalloc_ptr

This changes ui::input_handler to take a unique_xmalloc_ptr.  This
clarifies the ownership transfer of input_handler's argument.

gdb/ChangeLog
2018-12-30  Tom Tromey  <tom@tromey.com>

	* event-top.h (command_line_handler): Update.
	* top.c (class gdb_readline_wrapper_cleanup) <m_handler_orig>:
	Update.
	(gdb_readline_wrapper_line): Update.
	* top.h (struct ui) <input_handler>: Take a unique_xmalloc_ptr.
	(handle_line_of_input): Update.
	* event-top.c: Update.
	(gdb_readline_no_editing_callback): Update.
	(command_line_handler): Take a unique_xmalloc_ptr.
	(handle_line_of_input): Take a const char *.
	(command_line_append_input_line): Take a const char *.