libxl: Protect fds with CLOEXEC even with forking threads

libxl: Protect fds with CLOEXEC even with forking threads

We introduce a new "carefd" concept, which relates to fds that we care
about not being inherited by long-lived children.

As yet we do not use this anywhere in libxl.  Until all locations in
libxl which make such fds are converted, libxl__postfork may not work
entirely properly.  If these locations do not use O_CLOEXEC (or use
calls for which there is no O_CLOEXEC) then multithreaded programs may
not work properly.

This introduces a new API call libxl_postfork_child_noexec which must
be called by applications which make long-running non-execing
children.  Add the appropriate call to xl's postfork function.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>