New function SB-EXT:ASSERT-VERSION->=

Executing (sb-ext:assert-version->= 1 1 13) signals a continuable
error if the running SBCL is older than 1.1.13 (and an undefined
function error before that).

Based on a patch by Philip Munksgaard.

Closes lp#674372.
More reliable test for asynchronous aborts in semaphore operations

Test case by Andreas Franke (lp#1038034, comment #17).  We'll see
about committing the somewhat hairy fix for CONDITION-WAIT in #16
some other time.
More meaningful error message for OPEN :IF-EXISTS :NEW-VERSION

We don't try to simulate versioned filesystems on top of standard
UNIX operations, because that would be lossy.  Now explained more
explicitly.

Based on a patch by Philip Munksgaard, fixes lp#455381.