Defines OSSL_SSIZE_MAX

Removes SSIZE_MAX definition from bss_bio.c and changes that file to use
OSSL_SSIZE_MAX.

No need to account for OPENSSL_SYS_VXWORKS, since that never actually
gets defined anywhere. It must be a historical artifact.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Ben Laurie <ben@openssl.org>