In mi-lttng[.hc]: define strings related to command snapshot for further uses.

In snapshot.c: Add machine interface functionnality to command snapshot
In mi-lttng[.hc]: add mi_lttng_snapshot_output_session_name and
mi_lttng_snapshot_list_output to support the machine interface of
list_output in command snapshot.

In snapshot.c: add mi_list_output to support the machine interface
version of list-output.
In mi-lttng[.hc]: add mi_lttng_snapshot_del_output to support the machine 
interface of del_output in command snapshot.

In snapshot.c: add mi_del_output to support the machine interface
version of command snapshot del_output.
In mi-lttng[.hc]: add mi_lttng_snapshot_add_output to support the machine 
interface of add_output in command snapshot.

In snapshot.c: add mi_add_output to support the machine interface
version of command snapshot add_output.
In mi-lttng[.hc]: add mi_lttng_snapshot_record to support the machine
interface of record  in command snapshot.

In snapshot.c: add mi_record  to support the machine interface
version of command snapshot record.