Add GTSM support (RFC 6720).

Add GTSM support (RFC 6720).

This also finishes the missing bits from our RFC 7552 implementation
because GTSM is mandatory for LDPv6.

To avoid any kind of interoperability problems, I included a few
knobs to enable/disable GTSM on a per-address-family and per-neighbor
basis. Cisco's LDPv6 implementation, for instance, doesn't support GTSM.

"reads good" claudio@