tools/libs/*: Use O_CLOEXEC on Linux and FreeBSD

tools/libs/*: Use O_CLOEXEC on Linux and FreeBSD

In some cases this replaces an FD_CLOEXEC dance, in others it is new.

Linux has had O_CLOEXEC since 2.6.23 (October 2007), so we can rely on
it from Xen 4.7 I think. Some libc headers may still lack the
definition, so we take care of that if need be by defining to 0 (on
the premise that such an old glibc might barf on O_CLOEXEC even if the
kernel may or may not be so old).

All stable versions of FreeBSD support O_CLOEXEC (10.2, 9.3 and 8.4),
and we assume the libc there does too.

Remove various comments about having to take responsibility for this
(since really it is just hygiene, politeness, not a requirement) and
the reasons for using O_CLOEXEC seem pretty straightforward.

Backends for other OSes are untouched.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: Roger.Pau@citrix.com
Cc: jbeulich@suse.com