Transfer filter strings from sessiond to client

Transfer filter strings from sessiond to client

Previously, the payload of the list events was a simple array of
struct lttng_event.

    [event1][event2][event3]

This patch adds an array of variable-length extended infos at the
end of the event array:

    [event1][event2][event3][extinfo1][extinfo2][extinfo3]

When received on the client side, the event structures are
edited so that their new extended.ptr member points to their
respective extended infos:

    [event1][event2][event3][extinfo1][extinfo2][extinfo3]
        \       \       \      ^         ^         ^
         \       \       \____/_________/_________/
          \       \__________/_________/
           \________________/

This allows this whole block to be freed at once, so as to
keep existing applications linked with liblttng-ctl working
without being recompiled.

The variable-length extended info has this layout for the moment:

    [extended info header][filter string]

The extended info header contains the length of the subsequent
filter string, including its terminal null character.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>