Use the getprogname gnulib module

Make use of the recently added 'getprogname' module in gnulib: replace
our guestfs_int_program_name with the getprogname() provided by the
module, since it does the same thing, and in a portable way.
As consequence of the above, use gnulib in a couple of tests that use
getprogname().

Since guestfs_int_program_name is gone, drop the configure checks
associated with it.

rescue/rescue.c

@@ -32,6 +32,7 @@
 
 #include "ignore-value.h"
 #include "xvasprintf.h"
+#include "getprogname.h"
 
 #include "guestfs.h"
 #include "options.h"
@@ -56,7 +57,7 @@ usage (int status)
 {
   if (status != EXIT_SUCCESS)
     fprintf (stderr, _("Try `%s --help' for more information.\n"),
-             guestfs_int_program_name);
+             getprogname ());
   else {
     printf (_("%s: Run a rescue shell on a virtual machine\n"
               "Copyright (C) 2009-2016 Red Hat Inc.\n"
@@ -82,8 +83,8 @@ usage (int status)
               "  -w|--rw              Mount read-write\n"
               "  -x                   Trace libguestfs API calls\n"
               "For more information, see the manpage %s(1).\n"),
-            guestfs_int_program_name, guestfs_int_program_name,
-            guestfs_int_program_name, guestfs_int_program_name);
+            getprogname (), getprogname (),
+            getprogname (), getprogname ());
   }
   exit (status);
 }
@@ -279,7 +280,7 @@ main (int argc, char *argv[])
     fprintf (stderr, _("%s: error: extra argument '%s' on command line.\n"
              "Make sure to specify the argument for --format or --scratch "
              "like '--format=%s'.\n"),
-             guestfs_int_program_name, argv[optind], argv[optind]);
+             getprogname (), argv[optind], argv[optind]);
     usage (EXIT_FAILURE);
   }
 
@@ -288,7 +289,7 @@ main (int argc, char *argv[])
   /* User must have specified some drives. */
   if (drvs == NULL) {
     fprintf (stderr, _("%s: error: you must specify at least one -a or -d option.\n"),
-             guestfs_int_program_name);
+             getprogname ());
     usage (EXIT_FAILURE);
   }
 
@@ -306,7 +307,7 @@ main (int argc, char *argv[])
           STRPREFIX (backend, "libvirt:")) {
         fprintf (stderr, _("%s: warning: virt-rescue doesn't work with the libvirt backend\n"
                            "at the moment.  As a workaround, forcing backend = 'direct'.\n"),
-                 guestfs_int_program_name);
+                 getprogname ());
         if (guestfs_set_backend (g, "direct") == -1)
           exit (EXIT_FAILURE);
       }