Fix sysutils/zbackup in the face of modern C++

The "smart" pointer has had a serious bug for years, but the
method with the bug isn't used :S Fix the bug, throw in some
gratuitous C++ modernizations while we're at it.

Patches submitted upstream.

sysutils/zbackup/Makefile

@@ -2,7 +2,7 @@
 
 PORTNAME=	zbackup
 PORTVERSION=	1.4.4
-PORTREVISION=	15
+PORTREVISION=	16
 CATEGORIES=	sysutils
 
 MAINTAINER=	ports@FreeBSD.org

sysutils/zbackup/files/patch-sptr.hh

@@ -0,0 +1,47 @@
+--- sptr.hh.orig	2015-09-16 07:28:04 UTC
++++ sptr.hh
+@@ -25,9 +25,9 @@ class sptr_base
+ 
+ public:
+ 
+-  sptr_base(): p( 0 ), count( 0 ) {}
++  sptr_base(): p( nullptr ), count( nullptr ) {}
+ 
+-  sptr_base( T * p_ ): p( p_ ), count( p ? new unsigned( 1 ) : 0 )
++  sptr_base( T * p_ ): p( p_ ), count( p ? new unsigned( 1 ) : nullptr )
+   {
+   }
+ 
+@@ -48,27 +48,27 @@ class sptr_base
+       {
+         delete count;
+ 
+-        count = 0;
++        count = nullptr;
+ 
+         if ( p )
+         {
+           T * p_ = p;
+   
+-          p = 0;
++          p = nullptr;
+   
+           delete p_;
+         }
+       }
+       else
+       {
+-        p = 0;
+-        count = 0;
++        p = nullptr;
++        count = nullptr;
+       }
+     }
+   }
+ 
+   unsigned use_count() const
+-  { return count; }
++  { return *count; }
+ 
+   sptr_base & operator = ( sptr_base const & other )
+   { if ( &other != this ) { reset(); p = other.p; count = other.count; increment(); }