lttng-ctl: notifications: use epoll()/poll() instead of select()

The select(2) system call is an ancient ABI limited to processes
containing at most FD_SETSIZE file descriptors overall (typically
1024).

Those notification APIs will fail if the target file descriptor
is above FD_SETSIZE in a process containing many file descriptors.

Never use select, use the lttng epoll/poll wrapper instead.

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