Gzip: fixed setting of NGX_HTTP_GZIP_BUFFERED.

In r2411 setting of NGX_HTTP_GZIP_BUFFERED in c->buffered was moved from
ngx_http_gzip_filter_deflate_start() to ngx_http_gzip_filter_buffer() since
it was always called first.  But in r2543 the "postpone_gzipping" directive
was introduced, and if postponed gzipping is disabled (the default setting),
ngx_http_gzip_filter_buffer() is not called at all.

We must always set NGX_HTTP_GZIP_BUFFERED after the start of compression
since there is always a trailer that is buffered.

There are no known cases when it leads to any problem with current code.
But we already had troubles in upcoming SPDY implementation.
Removed unused prototype of ngx_http_find_server_conf().

This function prototype and its implementation was added in r90,
but the implementation was removed in r97.
Fixed logging in ngx_http_wait_request_handler().

If c->recv() returns 0 there is no sense in using ngx_socket_errno for
logging, its value meaningless.  (The code in question was copied from
ngx_http_keepalive_handler(), but ngx_socket_errno makes sense there as it's
used as a part of ECONNRESET handling, and the c->recv() call is preceeded
by the ngx_set_socket_errno(0) call.)