s_client/s_server: support unix domain sockets

The "-unix <path>" argument allows s_server and s_client to use a unix
domain socket in the filesystem instead of IPv4 ("-connect", "-port",
"-accept", etc). If s_server exits gracefully, such as when "-naccept"
is used and the requested number of SSL/TLS connections have occurred,
then the domain socket file is removed. On ctrl-C, it is likely that
the stale socket file will be left over, such that s_server would
normally fail to restart with the same arguments. For this reason,
s_server also supports an "-unlink" option, which will clean up any
stale socket file before starting.

If you have any reason to want encrypted IPC within an O/S instance,
this concept might come in handy. Otherwise it just demonstrates that
there is nothing about SSL/TLS that limits it to TCP/IP in any way.

(There might also be benchmarking and profiling use in this path, as
unix domain sockets are much lower overhead than connecting over local
IP addresses).

Signed-off-by: Geoff Thorpe <geoff@openssl.org>

e_os.h

@@ -579,6 +579,16 @@ static unsigned int _strlen31(const char *str)
 #      include <inet.h>
 #    else
 #      include <sys/socket.h>
+#      ifndef NO_SYS_UN_H
+#        ifdef OPENSSL_SYS_VXWORKS
+#          include <streams/un.h>
+#        else
+#          include <sys/un.h>
+#        endif
+#        ifndef UNIX_PATH_MAX
+#          define UNIX_PATH_MAX sizeof(((struct sockaddr_un *)NULL)->sun_path)
+#        endif
+#      endif
 #      ifdef FILIO_H
 #        include <sys/filio.h> /* Added for FIONBIO under unixware */
 #      endif