DragonFly BSD supports SCM_CREDS and SCM_RIGHTS, but only the first control
message is passed correctly while the second one isn't processed by the kernel.

This closes #599 issue on GitHub.

A prototype stores linked application processes structures.  When an
application process terminates, it's removed from the list.  To avoid double
removal, the pointer to the next element should be set to NULL.

The issue was introduced in c8790d2a89bb.

In PHP, custom fastcgi_finish_request() and overloaded chdir() functions can be
invoked by an OPcache preloading script (it runs when php_module_startup() is
called in the app process setup handler).  In this case, there was no runtime
context set so trying to access it caused a segmentation fault.

This closes #602 issue on GitHub.

The reference to the access_log structure is stored in the current
nxt_router_conf_t and the global nxt_router_t.  When the reference is copied,
the reference counter should be adjusted accordingly.

This closes #593 issue on GitHub.