Export session identity to the sessioncontrol hooks

[jslove]

command_start() and command_stop() run the sessioncontrol commands
(run_this_before_play_begins / run_this_after_play_ends) via fork and
execv of a fixed command string: no arguments, and nothing about the
session that triggered them. A hook can therefore be told that play
began, but not by whom -- which makes it impossible to distinguish
independent streams from several receivers carrying one grouped
AirPlay 2 stream.

Export the session's identity into the environment immediately before
running each hook. The child inherits it across fork/execv, so neither
the hook command string nor command_start()/command_stop() need to
change:

SPS_CLIENT_IP conn->client_ip_string
SPS_CONNECTION_NUMBER conn->connection_number
SPS_GROUP_ID conn->airplay_gid, the AirPlay 2 group UUID
from the SETUP plist ("" if none)
SPS_GROUP_CONTAINS_LEADER conn->groupContainsGroupLeader

The last two are set to ""/0 on non-AirPlay 2 builds so that a hook can
rely on all four always being present. Nothing inside shairport-sync
reads these; behaviour is unchanged for any configuration that does not
use sessioncontrol hooks.

[mikebrady]

This is lovely, thanks.