crypto/bio/b_dump.c: change all char* to void*, and constify

Some of these functions take char*, which is seldom right, they should
have been unsigned char*, because the content isn't expected to be
text.

Even better is to simply take void* as data type, which also happens
to be transparent for any type these functions are called with, be it
char* or unsigned char*.  This shouldn't break anything.

While we're at it, constify the input data parameters.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7890)