Transfer UST exclusion names from sessiond to client

The variable-length extended info has this layout as of this
patch:

    [extended info header][filter string][excl name 1][excl name 2]...

The extended info header contains the length of the subsequent
filter string, including its terminal null character, as well as
the number of exclusion names, each one having a fixed length of
LTTNG_SYMBOL_NAME_LEN bytes, including their terminal null character.

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