sb-bsd-sockets: More robust inet-socket-bind test on Windows.

Nested unwind-protects aren't supported on Windows.
Factor out read-var-integer into a function.

read-var-integer macro is used quite a number of times, expand the
macro into a SETF, which calls %read-var-integer, which does actual
reading. Reduces the core size by 65KB on x86-64.