Major fix 64-bit un_shlib_1() and unpack(), but not --android-shlib

Also canUnpack() is really a 'bool' which sets overlay_offset
when 'true'; confusing!
        https://github.com/upx/upx/issues/599 (partial)
	modified:   p_lx_elf.cpp
	modified:   p_lx_elf.h
	modified:   p_unix.h

src/p_unix.h

@@ -49,7 +49,7 @@ public:
     virtual void unpack(OutputFile *fo) override;
 
     virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual int  canUnpack() override;  // really 'bool'
     int find_overlay_offset(MemBuffer const &buf);
 
 protected: