Fix: sessiond: abort called on undefined client command

Issue observed
--------------

When running in verbose mode, the session daemon calls abort()
when it receives an unknown client command:

 #1  0x00007f66ffd69958 in raise () from /usr/lib/libc.so.6
 #2  0x00007f66ffd5353d in abort () from /usr/lib/libc.so.6
 #3  0x000055a671a6f6bd in lttcomm_sessiond_command_str (cmd=1633771873)
     at ../../../src/common/sessiond-comm/sessiond-comm.hpp:199
 #4  0x000055a671a73897 in process_client_msg (cmd_ctx=0x7f66f5ff6d10,
     sock=0x7f66f5ff6c34, sock_error=0x7f66f5ff6c38) at client.cpp:1006
 #5  0x000055a671a777fc in thread_manage_clients (data=0x55a673956100)
     at client.cpp:2622
 #6  0x000055a671a6d290 in launch_thread (data=0x55a673956170) at thread.cpp:68

Cause
-----

process_client_msg() logs the client command on entry. While it
previously logged the numerical value, it now provides the string-ified
version of the command id (since 19f912db8).

The lttcomm_sessiond_command_str() function aborts when it encounters an
unknown command id. This is reasonable (in so far that it is how we
handle these situations, typically). However, the validity of the
command must be checked beforehand as it comes from an
external (untrusted) source.

Solution
--------

Add lttcomm_sessiond_command_is_valid and tombstone command IDs to
lttcomm_sessiond_command to ensure only valid command ids are passed
to lttcomm_sessiond_command_str when logging.

Drawbacks
---------

None

Reported-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ibd36f1e69da984c7f27b55ec68e5e3fe06d7ac91