#3875 sign(z) for complex z should be an error

[Stavros Macrakis]

declare(z,complex)$
sign(z) => pnz
sign(sqrt(z)) => pz

These are incorrect; they should both give an error the way sign(%i) does.

csign is correct:

declare(z,complex)$
csign(z) => complex
csign(sqrt(z)) => complex