netsniff-ng: new package

[Thomas:
 - add comment in Config.in to explain why this package depends on
   glibc, and specifically why it doesn't build for uClibc and musl.
 - add hash file.
 - add 'COPYING' to the list of LICENSE_FILES
 - add themissing <pkg>_DEPENDENCIES variable in the .mk file, to make
   sure dependencies are built before netsniff-ng.
 - add comment in the .mk file explain why we're using the
   generic-package infrastructure even if the build procedure is
   essentially ./configure, make, make install.
 - fix indentation in NETSNIFF_NG_CONFIGURE_CMDS.]

Signed-off-by: Joris Lijssens <joris.lijssens@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>