Merge of r5017: fixed null dereference with resolver and poll.

Events: fixed null pointer dereference with resolver and poll.

A POLLERR signalled by poll() without POLLIN/POLLOUT, as seen on
Linux, would generate both read and write events, but there's no
write event handler for resolver events.  A fix is to only call
event handler of an active event.
Merge of r5018: secure link: fixed configuration inheritance.

The "secure_link_secret" directive was always inherited from the outer
configuration level even when "secure_link" and "secure_link_md5" were
specified on the inner level.
Merge of r5027, r5028, r5029: fastcgi_keep_conn fixes.

*) FastCGI: fixed wrong connection close with fastcgi_keep_conn.

   With fastcgi_keep_conn it was possible that connection was closed after
   FCGI_STDERR record with zero padding and without any further data read
   yet.  This happended as f->state was set to ngx_http_fastcgi_st_padding
   and then "break" happened, resulting in p->length being set to
   f->padding, i.e. 0 (which in turn resulted in connection close).

   Fix is to make sure we continue the loop after f->state is set.

*) FastCGI: unconditional state transitions.  Checks for f->padding
   before state transitions make code hard to follow, remove them and
   make sure we always do another loop iteration after f->state is
   set to ngx_http_fastcgi_st_padding.

*) FastCGI: proper handling of split fastcgi end request.  If fastcgi
   end request record was split between several network packets, with
   fastcgi_keep_conn it was possible that connection was saved in
   incorrect state (e.g. with padding bytes not yet read).