For providers that link with a shared libcrypto on Windows, it's necessary
to copy the OpenSSL DLLs to the directory where the provider resides, 'cause
that's how Windows works.  To that end, this macro now also provides the
following variables if they aren't already defined:

OPENSSL_RUNTIME_DIR             The runtime directory, i.e. where openssl.exe
                                resides
OPENSSL_LIBCRYPTO_SHARED        The libcrypto DLL file name
OPENSSL_LIBSSL_SHARED           The libssl DLL file name

This is not provided for non-Windows platforms, as it's not necessary there.

A lot of effort went into trying to fix FindOpenSSL.cmake's mixup of custom
and Shining Light OpenSSL installations on Windows.  Unfortunately, that
ended up creating more of a mess.  It's therefore time to admit defeat.

The 'setup_provider_openssl' macro still has a purpose, though, to fill in
the missing parts, such as library and modules directories, the path of the
openssl program, etc.

With MSVC, if you know that the Shining Light distribution is installed, and
you want to try to use a custom install, simply do not build with the Debug
configuration, because that's what FindOpenSSL.com doesn't get right.  All
other configurations should use the custom installation's libraries.

Note: the information given here relies on the library target property
IMPORTED_LOCATION_DEBUG only being used for the Debug configuration,
while the property IMPORTED_LOCATION_RELEASE is used for all other
configurations.

The idea is to pre-generate a parse-tree for all params in a single
provider.  This implementation creates a TRIE from all the param keys,
which are specified in a file, and implements it as a single function
(TRIE-as-code) that returns a parameter key index, along with a set of
macros that give names to those indexes.

Its use is detected with the presence of the variable 'OpenSSL_CONFIG'.
If it isn't, an attempt is made to compensate for what FindOpenSSL.cmake
doesn't get quite right, or where it's simply lacking.

This allows other packages to make additional preparations before using
the build_provider macro

Correct OPENSSL_APPLINK_SOURCE and OpenSSL::applink if they aren't set.
Correct not only OPENSSL_CRYPTO_LIBRARIES and OPENSSL_SSL_LIBRARIES,
but also OPENSSL_CRYPTO_LIBRARIES and OPENSSL_SSL_LIBRARY.

[work in progress]

This leaves it to different platforms to apply an appropriate prefix
and suffix according to their standards.

Related: https://github.com/gost-engine/engine/pull/313

If another package add_subdirectory() this one, and found OpenSSL first,
it's pointless to do the whole work again.

It was named 'prov', is now renamed to 'libprov'