In share package decfp, declare user-visible functions with DEFMFUN instead of DEFUN.
(DEFMFUN $FOO ...) defines two functions, one named $FOO and the other FOO-IMPL.
A subsequent (DEFUN $FOO ...) redefines only $FOO and leaves FOO-IMPL untouched.
decfp redefines $BFLOAT and the presence of the old BFLOAT-IMPL leads
to unexpected behavior.
Partial fix for SF bug #4110: "share package decfp inadvertent interaction with floor(sqrt(2))"
With this commit, floor(sqrt(2)) returns floor(sqrt(2)) which is still incorrect;
it returns 1 when decfp is not loaded.