Rename rl_callback_read_char_wrapper -> gdb_rl_callback_read_char_wrapper

Rename rl_callback_read_char_wrapper -> gdb_rl_callback_read_char_wrapper

Use the "gdb_rl_" prefix like other gdb readline function wrappers to
make it clear this is a gdb function, not a readline function.

gdb/ChangeLog:
2016-04-22  Pedro Alves  <palves@redhat.com>

	* event-top.c (rl_callback_read_char_wrapper): Rename to ...
	(gdb_rl_callback_read_char_wrapper): ... this.
	(change_line_handler, gdb_setup_readline): Adjust.