Do not use `bool` type; use new `bt_bool` instead

There are possible ABI incompatibilities between C99's `bool` (`_Bool`)
type and C++'s `bool` type, and also between different compilers. For
this reason we choose to use our own boolean type, `bt_bool`, which is
defined as `int`. `BT_FALSE` and `BT_TRUE` are our own definitions for
false and true.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>