Merge of r4985, r4986, r4987, r4988, r4989, r5002: access_log gzip.

*) Access log: fixed redundant buffer reallocation.  Previously a new
   buffer was allocated for every "access_log" directive with the same
   file path and "buffer=" parameters, while only one buffer per file
   is used.

*) Reopening log files code moved to a separate function.  The code
   refactored in a way to call custom handler that can do appropriate
   cleanup work (if any), like flushing buffers, finishing compress
   streams, finalizing connections to log daemon, etc..

*) Access log: the "flush" parameter of the "access_log" directive.

*) Configure: added the NGX_ZLIB define.  This was introduced for
   conditional compilation of the code that requires the zlib library.

*) Access log: the "gzip" parameter of the "access_log" directive.
   Note: this requires zlib version 1.2.0.4 or above to work.

*) The data pointer in ngx_open_file_t objects must be initialized.
   Uninitialized pointer may result in arbitrary segfaults if access_log
   is used without buffer and without variables in file path.
   Patch by Tatsuhiko Kubo (ticket #268).
Merge of r4991: fixed proxied HEAD requests with gzip enabled.

Fixed HEAD requests handling when proxying is used (ticket #261).
Merge of r4992: off-by-one with 32/64 upstream backup servers.

Fixed off-by-one during upstream state resetting when switching to
backup servers if there were exactly 32 (64 on 64-bit platforms)
backup servers configured.

Based on patch by Thomas Chen (ticket #257).
Merge of r4999, r5003: detect duplicate "events" and "keepalive".

*) Upstream keepalive: detect duplicate "keepalive" directive.  A
   failure to detect duplicate "keepalive" directive resulted in
   stack exhaustion.

*) Events: added check for duplicate "events" directive.
Merge of r5001: year 2013.
Merge of r5004, r5019-r5025: ssl fixes.

*) SSL: speedup loading of configs with many ssl servers.  The patch
   saves one EC_KEY_generate_key() call per server{} block by informing
   OpenSSL about SSL_OP_SINGLE_ECDH_USE we are going to use before
   the SSL_CTX_set_tmp_ecdh() call.

   For a configuration file with 10k simple server{} blocks with SSL
   enabled this change reduces startup time from 18s to 5s on a slow
   test box here.

*) SSL: removed conditions that always hold true.

*) SSL: resetting of flush flag after the data was written.  There is
   no need to flush next chunk of data if it does not contain a buffer
   with the flush or last_buf flags set.

*) SSL: preservation of flush flag for buffered data.  Previously,
   if SSL buffer was not sent we lost information that the data
   must be flushed.

*) SSL: calculation of buffer size moved closer to its usage.
   No functional changes.

*) SSL: avoid calling SSL_write() with zero data size.  According to
   documentation, calling SSL_write() with num=0 bytes to be sent
   results in undefined behavior.

   We don't currently call ngx_ssl_send_chain() with empty chain and
   buffer.  This check handles the case of a chain with total data size
   that is a multiple of NGX_SSL_BUFSIZE, and with the special buffer
   at the end.

   In practice such cases resulted in premature connection close and
   critical error "SSL_write() failed (SSL:)" in the error log.

*) SSL: take into account data in the buffer while limiting output.
   In some rare cases this can result in a more smooth sending rate.

*) SSL: fixed ngx_ssl_handshake() with level-triggered event methods.
   Missing calls to ngx_handle_write_event() and ngx_handle_read_event()
   resulted in a CPU hog during SSL handshake if an level-triggered event
   method (e.g. select) was used.
Merge of r5012: configure: removed redundant variable assignment.
Merge of r5013: proxy: fixed proxy_method to always add space.

Before the patch if proxy_method was specified at http{} level the code
to add trailing space wasn't executed, resulting in incorrect requests
to upstream.
Merge of r5014, r5015, r5016, r5030: geoip ipv6 support.

*) Configure: fixed style of include directories.

*) Configure: fixed GeoIP library detection.

*) GeoIP: IPv6 support.  When using IPv6 databases, IPv4 addresses are
   looked up as IPv4-mapped IPv6 addresses.  Mostly based on a patch
   by Gregor Kališnik (ticket #250).

*) GeoIP: removed pseudo-support of "proxy" and "netspeed" databases.