Turn sowriteable(), sballoc() and sbfree() macro to inline functions.

Turn sowriteable(), sballoc() and sbfree() macro to inline functions.

soreadable() is already presented as inline function, but corresponding
sowriteable() is still macro. Also it's no reason to keep sballoc() and
sbfree() as macro.

The first argument of sballoc() and sbfree() is not used, but keep it for
a while.

ok kn@ bluhm@