Expose remote L2TP hostname to pppd scripts

[gldrk]

This is set by hostname in xl2tpd.conf and is separate from the name used during PPP's optional authentication. The latter is already exposed by pppd as PEERNAME. The host name (attribute type 7) is a mandatory part of the Control Connection Protocol and is thus very useful for the purposes of identification.
The most sensible way to implement this would be via an environment variable, using pppd's set option. This also applies to the remote IP address that is currently exposed through ipparam.