Unit router process may send mmap file decritptor to the application process
for further information exchange. During this process there may be various
errors, which should be described in application error log. If file descriptor
cannot be properly transferred with 'new mmap' message, fd variable will
be assigned to -1 and further syscalls using this file descriptor will fail.
For 'new port' message fd is checked in the same way.
This commit adds early 'invalid file descriptor' diagnostic and write
corresponding message to error log.
Found by Coverity (CID 308515).
For the optimization purpose, function nxt_unit_remove_process() expects
lib->mutex to be locked. The function then moves ports queue into temporary
queue and releases mutex. In nxt_unit_done() there were two errors: mutex was
not locked before nxt_unit_remove_process() call and mutex was not destroyed.
It is hard to tell what was possible negative impact of this errors.
Found by Coverity (CID 308517).
