The module implements random load-balancing algorithm with optional second
choice.  In the latter case, the best of two servers is chosen, accounting
number of connections and server weight.

Example:

upstream u {
    random [two [least_conn]];

    server 127.0.0.1:8080;
    server 127.0.0.1:8081;
    server 127.0.0.1:8082;
    server 127.0.0.1:8083;
}

The module allows passing requests to upstream gRPC servers.
The module is built by default as long as HTTP/2 support is compiled in.
Example configuration:

    grpc_pass 127.0.0.1:9000;

Alternatively, the "grpc://" scheme can be used:

    grpc_pass grpc://127.0.0.1:9000;

Keepalive support is available via the upstream keepalive module.  Note
that keepalive connections won't currently work with grpc-go as it fails
to handle SETTINGS_HEADER_TABLE_SIZE.

To use with SSL:

    grpc_pass grpcs://127.0.0.1:9000;

SSL connections use ALPN "h2" when available.  At least grpc-go works fine
without ALPN, so if ALPN is not available we just establish a connection
without it.

Tested with grpc-c++ and grpc-go.

The Accept-Encoding, Accept-Language, and User-Agent header fields
are now copied from the original request to pushed requests.

The phase is added instead of the try_files phase.  Unlike the old phase, the
new one supports registering multiple handlers.  The try_files implementation is
moved to a separate ngx_http_try_files_module, which now registers a precontent
phase handler.

Instead of turning off some randomly selected http modules
when --without-http is specified, just don't process the
whole http modules section.

Some combinations of options might cause the builds with the
--with-stream option to break due to invalid value of the
STREAM_INCS make variable, e.g.

auto/configure \
    --with-stream \
    --with-http_perl_module=dynamic \
    --without-http_memcached_module \
    --without-http_empty_gif_module \
    --without-http_browser_module \
    --without-http_upstream_hash_module \
    --without-http_upstream_ip_hash_module \
    --without-http_upstream_least_conn_module \
    --without-http_upstream_keepalive_module \
    --without-http_upstream_zone_module \

Explicit initialization of ngx_module_libs and ngx_module_link
matches what we already do when processing mail modules, and
is also required after the next change.

Dependencies of dynamic modules are added to NGX_ADDON_DEPS (and
it is now used for dynamic modules) to be in line with what happens
in case of static compilation.

To avoid duplication, MAIL_DEPS and STREAM_DEPS are no longer passed
to auto/module when these modules are compiled as dynamic ones.  Mail
and stream dependencies are handled explicitly via corresponding
variables.