Bluetooth: Add set_connectable management command

This patch adds a set_connectable command as well as a corresponding
event to the management interface. It's mainly useful for setting an
adapter as connectable from a non-initialized state as well as setting
an already initialized adapter as non-connectable (mostly useful for
qualification purposes).

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>