Remove internal functions OPENSSL_strcasecmp and OPENSSL_strncasecmp

Their only reason to exist was that they didn't exist in VMS before
version 7.0.  We do not support such old versions any more.

However, for the benefit of systems that don't get strings.h included
by string.h, we include the former in e_os.h.

RT#4458

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>