Add standalone mptcpd plugin example.

[ossama-othman]

Add a standalone (out-of-tree) mptcpd no-op plugin example to be used as a reference for mptcpd plugin developers.

[ossama-othman]

@mjmartineau I personally don't think it's good to build against the source tree of a dependency, but here's one way to build the sample plugin against a source tree without modifying Makefiles:

ELL_DIR=/path/to/ell \
MPTCPD_DIR=/path/to/mptcpd \
PKG_CONFIG_PATH=$ELL_DIR/ell:$MPTCPD_DIR/lib \
CPPFLAGS="-I$MPTCPD_DIR/include -I$ELL_DIR" \
LDFLAGS="-L$MPTCPD_DIR/lib -L$ELL_DIR/ell" \
./configure 

Regarding the environment variables, the sample plugin's configure script help output mentions several besides the usual preprocessor, compiler and linker flags that could be used for building against an mptcpd source tree:

  PKG_CONFIG  path to pkg-config utility
  PKG_CONFIG_PATH
              directories to add to pkg-config's search path
  PKG_CONFIG_LIBDIR
              path overriding pkg-config's built-in search path
  MPTCPD_CFLAGS
              C compiler flags for MPTCPD, overriding pkg-config
  MPTCPD_LIBS linker flags for MPTCPD, overriding pkg-config
  MPTCPD_PLUGINDIR
              value of plugindir for mptcpd, overriding pkg-config

ELL related variables would normally be pulled in through the mptcpd variables obtained through pkg-config, which is why they aren't listed in the sample plugin's configure script help output.

[ossama-othman]

Getting mptcpd to pick up new installed plugin currently requires a restart of mptcpd. I'll document that in the plugins wiki, too.