lib: enable deprecation warnings by default

Right now, deprecated functions of the library do not trigger any
compiler deprecation warning by default; they do that only if
GUESTFS_WARN_DEPRECATED=1 is defined.  However, this is not something
that seems to be done often -- at least none of the projects using the
libguestfs C API does that.

Hence, do a small behaviour change to change this on the other way
round: now deprecated functions trigger compiler deprecation warnings by
default, using GUESTFS_NO_WARN_DEPRECATED to disable this (and revert
to the previous behaviour).  Even though deprecated functions will not
be removed, we really want users to migrate away from them, as they were
deprecated for good reasons.

Define GUESTFS_NO_WARN_DEPRECATED where needed:
- in all the bindings, as they bind all the functions including the
  deprecated ones
- in the guestfish actions, as it exposes almost all the APIs
- in the C API test, as it runs the automated tests of all the APIs that
  have them
- for two tests that explicitly test deprecated functions

generator/OCaml.ml

@@ -412,6 +412,9 @@ and generate_ocaml_c () =
   pr "\
 #include <config.h>
 
+/* It is safe to call deprecated functions from this file. */
+#define GUESTFS_NO_WARN_DEPRECATED
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>