Document issue with default installation paths on diverse Windows targets

Document issue with default installation paths on diverse Windows targets

For all config targets (except VMS, because it has a completely different
set of scripts), '/usr/local/ssl' is the default prefix for installation
of programs and libraries, as well as the path for OpenSSL run-time
configuration.

For programs built to run in a Windows environment, this default is
unsafe, and the user should set a different prefix.  This has been hinted
at in some documentation but not all, and the danger of leaving the
default as is hasn't been documented at all.

This change documents the issue as a caveat lector, and all configuration
examples now include an example --prefix.

CVE-2019-1552

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9456)