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.

After the c8790d2a89bb commit, the SIGCHLD handler may return before processing
all awaiting PIDs.  To avoid zombie processes and ensure successful main
process termination, waitpid() must be called until an error is returned.

This closes #600 issue on GitHub.

Splitting the process type connectivity matrix to 'keep ports' and 'send
ports'; the 'keep ports' matrix is used to clean up unnecessary ports after
forking a new process, and the 'send ports' matrix determines which process
types expect to get created process ports.

Unfortunately, the original single connectivity matrix no longer works because
of an application stop delay caused by prototypes.  Existing applications
should not get the new router port at the moment.

The "recv_msg.incoming_buf" is checked after jumping to the "done" label
if nxt_socket_msg_oob_get_fds() returns an error.

Also moved initialization of "port_msg" near to its first usage.

Found by Coverity (CID 373899).

It was accidentally added in 4645a43bc248.

This enables the reuse of process creation functions.