Fix: Use unix socket peercred for pid, uid, gid credentials

Fix: Use unix socket peercred for pid, uid, gid credentials

Currently, the session daemon trust the pid, ppid, uid, and gid values
passed by the application, but should really validate the uid using unix
socket peercred. This fix uses the peercred values rather than the
values provided by the application on registration for:

- pid, uid and gid on Linux,
- uid and gid on FreeBSD.

This should improve how the session daemon deals with containerized
applications on Linux as well. Applications are required to be either in
the same pid namespace, or in a pid namespace nested within the pid
namespace of the lttng-sessiond, so the session daemon can map the
application pid to something meaningful within its own pid namespace.
Applications in a unrelated (disjoint) pid namespace will be refused by
the session daemon.

About the uid and gid with user namespaces on Linux, those will provide
meaningful IDs if the application user namespace is either the same as
the user namespace of the session daemon, or a nested user namespace.
Otherwise, the IDs will be that of /proc/sys/kernel/overflowuid and
/proc/sys/kernel/overflowgid, which typically maps to nobody.nogroup on
current distributions.

Given that fetching the parent pid (ppid) of the application would
require to use /proc/<pid>/status (which is racy wrt pid reuse), expose
the ppid provided by the application on registration instead, but only
in situations where the application sits in the same pid namespace as
the session daemon (on Linux), which is detected by checking if the pid
provided by the application matches the pid obtained using unix socket
credentials. The ppid is only used for logging/debugging purposes in the
session daemon anyway, so it is OK to use the value provided by the
application for that purpose.

Fixes: #1286
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I94742e57dad642106908d09e2c7e395993c2c48f