Introduction of sequential-API build variant, better thread safety (lwIP only)

ext/lwip-contrib/ports/unix/unixsim/README

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+This directory contains an example of how a project using lwIP might
+look. It is also the development platform of lwIP, since it can be run
+as a user process under FreeBSD or Linux. There are also a number of
+example applications (including a simple web server) in the apps/
+directory.
+
+Some short instructions on how to build and run lwIP on a FreeBSD or
+Linux host. For FreeBSD, the tap interface must be enabled in the
+kernel configuration and the kernel must be recompiled. The tap
+interface is enabled by adding the line "pseudo-device tap" in the
+kernel configuration. See Chapter 9 in the FreeBSD handbook for
+instructions on how to build a custom FreeBSD kernel.
+
+For Linux you might need to create a device node with
+
+  > mknod /dev/net/tun c 10 200
+
+* Compile the code. This must be done by using GNU Make. Under
+  FreeBSD, GNU Make can be found in the ports collection under
+  /usr/ports/devel/gmake (type "make install distclean" to
+  install). Under Linux, GNU Make is the default "make".
+
+  > gmake         (FreeBSD)
+
+  > make          (Linux)
+
+* The compilation process produces the executable file "simhost". To
+  run this, you have to be root.
+
+  > su            (Type password for the root account)
+  # ./simhost
+
+* The lwIP TCP/IP stack is now running with IP address
+  192.168.0.2. Some things that you can try:
+
+  To see the packets that are going to and from the lwIP stack, run
+  tcpdump:
+
+  # tcpdump -l -n -i tap0
+
+  You can ping lwIP:
+
+  > ping 192.168.0.2
+
+  For a telnet shell, run:
+
+  > telnet 192.168.0.2
+  
+  Finally, "simhost" also includes a simple web server; the URL is
+  of course http://192.168.0.2/.
+
+* Simhost has some extra features that can be
+  selected with command line options.
+  
+  To enable runtime debug output (project must be build with -DLWIP_DEBUG):
+  
+  # ./simhost -d
+  
+  To ping any host, e.g. the gateway:
+  
+  # ./simhost -p 192.168.0.1