Use MT-safe strtok_r in spawn viewer library helper

Use MT-safe strtok_r in spawn viewer library helper

Even though its current callers are all in single-threaded context
(lttng view and lttng-crash commands), it is a good practice to use
MT-safe APIs in library functions, in case those are ever used within a
multithreaded context in the future.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: If419fd24b3bf6532f2c75a728a9ec395e365c626