Fix strict aliasing issues

[ellert]

/builddir/build/BUILD/xrootd-4.2.3/src/XrdSys/XrdSysDNS.cc: In static member function 'static int XrdSysDNS::getAddrName(const char*, int, char**, char**, char**)':
/builddir/build/BUILD/xrootd-4.2.3/src/XrdSys/XrdSysDNS.cc:187:51: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
       int hn = getHostName((struct sockaddr &)ip[i], names, 1, errtxt);
                                                   ^
/builddir/build/BUILD/xrootd-4.2.3/src/XrdSys/XrdSysDNS.cc: In static member function 'static int XrdSysDNS::Host2Dest(const char*, sockaddr&, char**)':
/builddir/build/BUILD/xrootd-4.2.3/src/XrdSys/XrdSysDNS.cc:522:47: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
    if (!getHostAddr(hbuff, (struct sockaddr &)InetAddr, errtxt)) return 0;
                                               ^~~~~~~~
/builddir/build/BUILD/xrootd-4.2.3/src/XrdSys/XrdSysDNS.cc: In static member function 'static int XrdSysDNS::Host2IP(const char*, unsigned int*)':
/builddir/build/BUILD/xrootd-4.2.3/src/XrdSys/XrdSysDNS.cc:550:47: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]
    if (!getHostAddr(hname, (struct sockaddr &)InetAddr)) return 0;
                                               ^~~~~~~~

[abh3]

Thank you Mattias for the fix. We thought we would do this using unions but your solution is just as nice :-)