Upgrade from 1.12.0 to latest "mainline" 1.13.0.

<ChangeLog>

*) Change: SSL renegotiation is now allowed on backend connections.

*) Feature: the "rcvbuf" and "sndbuf" parameters of the "listen"

directives of the mail proxy and stream modules.

*) Feature: the "return" and "error_page" directives can now be used to

return 308 redirections.
Thanks to Simon Leblanc.

*) Feature: the "TLSv1.3" parameter of the "ssl_protocols" directive.

*) Feature: when logging signals nginx now logs PID of the process which

sent the signal.

*) Bugfix: in memory allocation error handling.

*) Bugfix: if a server in the stream module listened on a wildcard

address, the source address of a response UDP datagram could differ
from the original datagram destination address.

</ChangeLog>