This makes it possible to avoid looping for a long time while working
with a fast enough peer when data are added to the socket buffer faster
than we are able to read and process them (ticket #1431).  This is
basically what we already do on FreeBSD with kqueue, where information
about the number of bytes in the socket buffer is returned by
the kevent() call.

With other event methods rev->available is now set to -1 when the socket
is ready for reading.  Later in ngx_recv() and ngx_recv_chain(), if
full buffer is received, real number of bytes in the socket buffer is
retrieved using ioctl(FIONREAD).  Reading more than this number of bytes
ensures that even with edge-triggered event methods the event will be
triggered again, so it is safe to stop processing of the socket and
switch to other connections.

Using ioctl(FIONREAD) only after reading a full buffer is an optimization.
With this approach we only call ioctl(FIONREAD) when there are at least
two recv()/readv() calls.

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.

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).

Determine cacheline size at runtime if supported
using sysconf(_SC_LEVEL1_DCACHE_LINESIZE). In case not supported,
fallback to compile time defaults.

In 2c7b488a61fb, IP_BIND_ADDRESS_NO_PORT test was accidentally placed
between SO_BINDANY, IP_TRANSPARENT, and IP_BINDANY tests.  Moved it after
these tests.

On Cygwin and NetBSD 7.0+ struct in_pktinfo has no ipi_spec_dst field, which
caused nginx compilation error.  Now presence of this field is ensured by the
IP_PKTINFO feature test.

The problem was introduced by dbb0c854e308 (1.13.0).

The sched_setaffinity() function was introduced in DragonFly BSD 4.7,
so it is no longer Linux-specific.

Prodded by Sepherosa Ziehau.

Previously, the source IP address of a response UDP datagram could differ from
the original datagram destination address.  This could happen if the server UDP
socket is bound to a wildcard address and the network interface chosen to output
the response packet has a different default address than the destination address
of the original packet.  For example, if two addresses from the same network are
configured on an interface.

Now source address is set explicitly if a response is sent for a server UDP
socket bound to a wildcard address.