This was previously used, but was incorrectly removed in 83d54192e97b
while removing old threads remnants.  Instead of using it conditionally
when threads are not used, we now set in unconditionally, as even with
thread pools enabled we never call OpenSSL functions in threads.

This fixes resulting binary when using --with-openssl with OpenSSL 1.1.0+
and without -lpthread linked (notably on FreeBSD without PCRE).

OpenSSL now uses pthread_atfork(), and this requires -lpthread on Linux
to compile.  Introduced NGX_LIBPTHREAD to add it as appropriate, similar
to existing NGX_LIBDL.

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.

When clock_gettime(CLOCK_MONOTONIC) (or faster variants, _FAST on FreeBSD,
and _COARSE on Linux) is available, we now use it for ngx_current_msec.
This should improve handling of timers if system time changes (ticket #189).

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

Previously, capset(2) was called with the 64-bit capabilities version
_LINUX_CAPABILITY_VERSION_3.  With this version Linux kernel expected two
copies of struct __user_cap_data_struct, while only one was submitted.  As a
result, random stack memory was accessed and random capabilities were requested
by the worker.  This sometimes caused capset() errors.  Now the 32-bit version
_LINUX_CAPABILITY_VERSION_1 is used instead.  This is OK since CAP_NET_RAW is
a 32-bit capability (CAP_NET_RAW = 13).

Previously included file sys/capability.h mentioned in capset(2) man page,
belongs to the libcap-dev package, which may not be installed on some Linux
systems when compiling nginx.  This prevented the capabilities feature from
being detected and compiled on that systems.

Now linux/capability.h system header is included instead.  Since capset()
declaration is located in sys/capability.h, now capset() syscall is defined
explicitly in code using the SYS_capset constant, similarly to other
Linux-specific features in nginx.

The capability is retained automatically in unprivileged worker processes after
changing UID if transparent proxying is enabled at least once in nginx
configuration.

The feature is only available in Linux.