Buffer for application prefork request allocated from temp conf mem_pool.
If error response from main process received before buffer completion handler,
temp conf mem_pool destroyed and router may crash in completion handler.

Assertion "src/nxt_buf.c:208 assertion failed: data == b->parent" triggered
when NXT_DEBUG_ALLOC enabled in configure.

This patch disables completion handler and memory allocated for buffer
released with memory pool.

Previously, an error during the prefork phase triggered assert:

 src/nxt_port.c:27 assertion failed: port->pair[0] == -1

and resulted in exiting of the main process.

This could be easily reproduced by pushing a configuration with "rootfs",
when daemon is running without required permissions.

Found by Coverity (CID 361277).

This closes #441 PR on GitHub.

After shared application port introducing, request queue in router was
removed and requests may stuck forever waiting for another process start.

Two consecutive fd and fd2 fields replaced with array.

The default libunit behavior relies on blocking the recv() call for port file
descriptors, which an application may override if needed.  For external
applications, port file descriptors were toggled to blocking mode before the
exec() call.  If the exec() call failed, descriptor remained blocked, so the
process hanged while trying to read from it.

This patch moves file descriptor mode switch inside libunit.