1.0.4.106: refactoring FILE-POSITION on FD-STREAMS, some cleanups

1.0.4.106: refactoring FILE-POSITION on FD-STREAMS, some cleanups

 * Make the underlying FILE-POSITION on FD-STREAMs interrupt-safe.
   This is not enough to make FILE-POSITION on FD-STREAMs interrupt
   safe, as the ANSI-STREAM layer is not -- or at least I doesn't look
   like it to me.

 * Split FD-STREAM-FILE-POSITION into two parts for easier reading,
   and make the error behaviour more ANSI compliant.

 * Move FLUSH-OUTPUT-BUFFER to FINISH-FD-STREAM-OUTPUT to make it a
   one-stop-shopping implementation of finish-output for FD-STREAMs.

 * New function: FD-STREAM-OUTPUT-FINISHED-P, which returns false if
   there is any pending output on the stream.

 * Add comments explaining why certain VECTOR-SAP usages are safe
   without pinning the vector -- at least on x86oids.

 * Instead of (IF #-WIN32 P #+WIN32 T #-WIN32 THEN #+WIN32 ELSE),
   let us use #-WIN32 ELSE #+WIN32 (IF P THEN ELSE)...