We intend to place a space character between the function designator and
the opening parenthesis in a function call. We've been executing on that
mostly consistently; fix the few exceptions now.

The same convention should be applied to the invocations of function-like
macros, and to instances of "__attribute__ ((attr))". (The latter is
exemplified, although not consistently, by the GCC manual.) Implement
this, by inserting the necessary spaces.

Furthermore, some compiler attributes take multiple parameters, such as
"nonnull". The GCC manual clearly shows that arguments passed to such
attributes may be separated with ", " as well, not just ",". Use the
former, as it's more readable.

Finally, the C standard calls "defined" -- as in "#if defined identifier"
and (equivalently) "#if defined (identifier)" -- a unary preprocessing
operator. We can spell the parenthesized form as "defined (identifier)"
rather than "defined(identifier)", so ...