Replumbing: add a configuration module for providers

This configuration module supports a configuration structure pretty
much like the engine configuration module, i.e. something like this:

    openssl_conf = openssl_init

    [openssl_init]
    providers = provider_section

    [provider_section]
    # Configure the provider named "foo"
    foo = foo_section
    # Configure the provider named "bar"
    bar = bar_section

    [foo_section]
    # Override name given in the provider section
    identity = myfoo
    # The exact path of the module.  This is platform specific
    module_path = /opt/openssl/modules/foo.so
    # Whether it should be automatically activated.  Value is unimportant
    activate = whatever

    # Anything else goes as well, and becomes parameters that the
    # provider can get
    what = 1
    # sub-sections will be followed as well
    ever = ever_section

    [ever_section]
    cookie = monster

All the configurations in a provider section and its sub-sections
become parameters for the provider to get, i.e. the "foo" provider
will be able to get values for the following keys (with associated
values shown):

    identity            => myfoo
    module_path         => /opt/openssl/modules/foo.so
    activate            => whatever
    what                => 1
    ever.cookie         => monster

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8549)