FastCGI: fixed start pointers in request buffers.

The start pointers are used in ngx_http_upstream_reinit() to
reinit FastCGI requests.
Upstream: fixed file buffers reinit in ngx_http_upstream_reinit().

Previously, a file buffer start position was reset to the file start.
Now it's reset to the previous file buffer end.  This fixes
reinitialization of requests having multiple successive parts of a
single file.  Such requests are generated by fastcgi module.
Fixed writev() debug log message in ngx_darwin_sendfile_chain().
Moved the code for adjusting sent buffers in a separate function.
Removed the "complete" variable from various send chain functions.

It was made redundant by the previous change, since the "sent" variable
is no longer modified.
Merged implementations of ngx_readv_chain().

There's no real need in two separate implementations,
with and without kqueue support.
Reduced difference between the send chain functions.

No functional changes.  This follows the change from ad137a80919f.
Generalized definitions of the number of preallocated iovec's.

No functional changes.
Removed duplicate initialization of the "rev" variable.
Avoided to add duplicate hash key in ngx_http_types_slot().
Syslog: enabled logging of send errors.

The ngx_cycle->log is used when sending the message.  This allows to log syslog
send errors in another log.

Logging to syslog after its cleanup handler has been executed was prohibited.
Previously, this was possible from ngx_destroy_pool(), which resulted in error
messages caused by attempts to write into the closed socket.

The "processing" flag is renamed to "busy" to better match its semantics.
Syslog: improved error handling of unix domain sockets.

If a syslog daemon is restarted and the unix socket is used, further logging
might stop to work.  In case of send error, socket is closed, forcing
a reconnection at the next logging attempt.
Limit conn: removed deprecated "limit_zone" directive.

It's deprecated since 260d591cb6a3 (1.1.8).  The "limit_conn_zone" directive
should be used instead.
Limit conn: use complex value in limit_conn_zone (ticket #121).

One intentional side effect of this change is that key is allowed only
in the first position.  Previously, it was possible to specify the key
variable at any position, but that was never documented, and is contrary
to nginx configuration practice for positional parameters.
Limit conn: aligned field names in structures.

No functional changes.
Limit req: use complex value in limit_req_zone.

One intentional side effect of this change is that key is allowed only
in the first position.  Previously, it was possible to specify the key
variable at any position, but that was never documented, and is contrary
with nginx configuration practice for positional parameters.
Limit req: reduced number of parameters in the lookup function.

No functional changes.
Upstream keepalive: reset c->sent on cached connections.

The c->sent is reset to 0 on each request by server-side http code,
so do the same on client side.  This allows to count number of bytes
sent in a particular request.
nginx-1.7.6-RELEASE
release-1.7.6 tag