Fix: run_command_wait() handle partial write

Use lttng_write() to handle partial writes (writing less than the
requested amount of bytes) as well as ret = -1, errno = EINTR.

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