libxl_internal.h says:

 * Functions using LIBXL__INIT_EGC may *not* generally be called from
 * within libxl, because libxl__egc_cleanup may call back into the
 * application. ...

and

 *                    ...  [Functions which take an ao_how] MAY NOT
 * be called from inside libxl, because they can cause reentrancy
 * callbacks.

However, this was not enforced.  Particularly the latter restriction
is easy to overlook, especially since during the transition period to
the new event system we have bent this rule a couple of times, and the
bad pattern simply involves passing 0 or NULL for the ao_how.

So use the compiler to enforce this property, as follows:

 - Mark all functions which take a libxl_asyncop_how, or which
   use EGC_INIT or LIBXL__INIT_EGC, with a new annotation
   LIBXL_EXTERNAL_CALLERS_ONLY in the public header.

 - Change the documentation comment for asynch operations and egcs to
   say that this should always be done.

 - Arrange that if libxl.h is included via libxl_internal.h,
   LIBXL_EXTERNAL_CALLERS_ONLY expands to __attribute__((warning(...))),
   which generates a message like this:
      libxl.c:1772: warning: call to 'libxl_device_disk_remove'
             declared with attribute warning:
             may not be called from within libxl
   Otherwise, the annotation expands to nothing, so external
   callers are unaffected.

 - Forbid inclusion of both libxl.h and libxl_internal.h unless
   libxl_internal.h came first, so that the above check doesn't have
   any loopholes.  Files which include libxl_internal.h should not
   include libxl.h as well.

   This is enforced explicitly using #error.  However, in practice
   with the current tree it just changes the error message when this
   mistake is made; otherwise we would carry on to immediately
   following #define which would cause the compiler to complain that
   LIBXL_EXTERNAL_CALLERS_ONLY was redefined.  Then the developer
   might be tempted to add a #ifndef which would be wrong - it would
   leave the affected translation unit unprotected by the new
   enforcement regime.  So let's be explicit.

 - Fix the one source of files which violate the above principle, the
   output from the idl compiler, by removing the redundant inclusion
   of libxl.h from the output.

Also introduce a new script "check-libxl-api-rules" which contains
some ad-hoc regexps to spot and complain when libxl.h contains
functions which mention libxl_asyncop_how but not
LIBXL_EXTERNAL_CALLERS_ONLY.  This isn't a full C parser but is likely
to get the common cases right and err on the side of complaining.

While we are here, the invocation of perl for the bsd queue.h seddery
to $(PERL).

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>