The PCRE2 library is now used by default if found, instead of the
original PCRE library.  If needed for some reason, this can be disabled
with the --without-pcre2 configure option.

To make it possible to specify paths to the library and include files
via --with-cc-opt / --with-ld-opt, the library is first tested without
any additional paths and options.  If this fails, the pcre2-config script
is used.

Similarly to the original PCRE library, it is now possible to build PCRE2
from sources with nginx configure, by using the --with-pcre= option.
It automatically detects if PCRE or PCRE2 sources are provided.

Note that compiling PCRE2 10.33 and later requires inttypes.h.  When
compiling on Windows with MSVC, inttypes.h is only available starting
with MSVC 2013.  In older versions some replacement needs to be provided
("echo '#include <stdint.h>' > pcre2-10.xx/src/inttypes.h" is good enough
for MSVC 2010).

The interface on nginx side remains unchanged.

Addon modules, both dynamic and static, can now use shared source files.
Shared sources result in only one make rule even if specified several
times in different modules.

Adds 'set' directive to the stream server context.

Postpone filter is an essential part of subrequest functionality.  In absence
of it a subrequest response body is sent to the client out of order with
respect to the main request header and body, as well as other subrequests.
For in-memory subrequests the response is also sent to the client instead of
being stored in memory.

Currently the postpone filter is automatically enabled if one of the following
standard modules which are known to create subrequests is enabled: ssi, slice,
addition.  However a third-party module that creates subrequests can still be
built without the postpone filter or be dynamically loaded in nginx built
without it.

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.

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