apparmor: userns: Add support for execpath in userns

apparmor: userns: Add support for execpath in userns

This new field allows reliable identification of the binary that
triggered a denial since the existing field (comm) only gives the name of
the binary, not its path. Thus comm doesn't work for binaries outside of
$PATH or works unreliably when two binaries have the same name.
Additionally comm can be modified by a program, for example, comm="(tor)"
or comm=4143504920506F6C6C6572 (= ACPI Poller).

Signed-off-by: Maxime Bélair <maxime.belair@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>