xdr_destroy and XDR_CONTROL should be expressions, not statements

[dmhayden]

xdr_destroy and XDR_CONTROL are defined as "if" statements in xdr.h, but they should be expressions.
#define xdr_destroy(xdrs)
if ((xdrs)->x_ops->x_destroy)
(*(xdrs)->x_ops->x_destroy)(xdrs)

As an "if" statement, xdr_destroy() can't appear in the increment clause of a "for" statement:
for (cond; test; xdr_destroy(xp)) { ... } // syntax error

Much worse, it can cause unexpected results when used within an if statement. Consider:
if (cond)
xdr_destroy(xdrs);
else
oops();

If xdrs->x_ops->x_destroy is NULL, then oops() will be called, which is not what the programmer would expect.