Previously, last_modified_time was tested against -1 to check if the
not modified filter should be skipped.  Notably, this prevented nginx
from additional If-Modified-Since (et al.) checks on proxied responses.
Such behaviour is suboptimal in some cases though, as checks are always
skipped on responses from a cache with ETag only (without Last-Modified),
resulting in If-None-Match being ignored in such cases.  Additionally,
it was not possible to return 412 from the If-Unmodified-Since if last
modification time was not known for some reason.

This change introduces explicit r->disable_not_modified flag instead,
which is set by ngx_http_upstream_process_headers().

See http://mailman.nginx.org/pipermail/nginx-devel/2013-November/004523.html.

log->filter ("if" parameter) was uninitialized when the default value
was being used, which would lead to a crash (SIGSEGV) when access_log
directive wasn't specified in the configuration.

Zero-fill the whole structure instead of zeroing fields one-by-one
in order to prevent similar issues in the future.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

In particular, properly output partial match at the end of a subrequest
response (much like we do at the end of a response), and reset/set the
last_in_chain flag as appropriate.

Reported by KAWAHARA Masashi.

Make two checks for maximum number of tries consistent.
The other one checks '>' condition.