sessiond: Add --extra-kmod-probes option

This patch adds the --extra-kmod-probes option to lttng-sessiond. The
LTTNG_EXTRA_KMOD_PROBES environment variable may also be used.

The option specifies a list of extra probe kernel modules to be loaded
(and unloaded) by lttng-sessiond. The list is appended to either the
default list or to the user-supplied --kmod-probes list.

This option is especially useful for kernel developers who need the
default LTTng kernel probes plus additional probes in order to
instrument their custom kernel or module. This becomes easy with
--extra-kmod-probes:

    lttng-sessiond --extra-kmod-probes=custom_subsys,other

would load all known and available LTTng kernel probes plus
lttng_probe_custom_subsys and lttng_probe_other.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>