[tls] Use intf_insert() to add TLS to an interface

[tls] Use intf_insert() to add TLS to an interface

Restructure the use of add_tls() to insert a TLS filter onto an
existing interface.  This allows for the possibility of using
add_tls() to start TLS on an existing connection (as used in several
protocols which will negotiate the choice to use TLS before the
ClientHello is sent).

Signed-off-by: Michael Brown <mcb30@ipxe.org>