Clarify limitations of the --syscall flag with enable-event command

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>
Backported to glibc 2.8

This patch enables lttng-tools to run on top of glibc 2.8.
Overall it fixes 2 things:
1. No support for epoll_create1(..) and EPOLL_CLOEXEC.
2. No support for htobe/betoh

For 1, we revert back to epoll_create() and then sets CLOEXEC through
fcntl instead.

For 2, we define htobe/betoh as part of the compat/endian.h and make
sure that any users of those functions actually include compat/endian.h
instead of implicit include of system endian.h

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tested-by: Jesper Derehag <jderehag@hotmail.com>
Signed-off-by: Jesper Derehag <jderehag@hotmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>