perl: Set program name to the true name instead of 'perl'.

Currently any Perl program has the handle program name field set
to 'perl'.  Set it to the true name (derived from $0).

generator/perl.ml

@@ -753,6 +753,12 @@ sub new {
   my $g = Sys::Guestfs::_create ($flags);
   my $self = { _g => $g, _flags => $flags };
   bless $self, $class;
+
+  # If we don't do this, the program name is always set to 'perl'.
+  my $program = $0;
+  $program =~ s{.*/}{};
+  $self->set_program ($program);
+
   return $self;
 }