Change comment to refer to "libffi"

Comment referred to "FSBV" now corrected to "libffi".  Other uses (the
tests and foreign library for tests, the *feature* symbol, and the
variable 'fsbvp resulting from fn-call-by-value-p) genuinely refer to
FSBV and not specifically libffi.  Distinction: "FSBV" refers to all
calls of structures by value, whether accomplished with the help of
libffi or not (some compilers have FSBV built in), "libffi" refers to
the use of the libffi library whether or not for calling foreign
structures by value (there are other functions not yet made use of in
CFFI, but someday might).