mount.nfs: Fix sockaddr pointer aliasing in stropts.c

Using a sockaddr_storage and casting a sockaddr pointer to it breaks
C's aliasing rules.

See:

  https://bugzilla.redhat.com/show_bug.cgi?id=448743

Replacing sockaddr_storage makes this code less likely to break when
optimized by gcc.  It also saves a significant amount of stack space
by replacing a 130 byte structure with a union that is less than 32
bytes.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>