Miscellaneous code cleaning

Miscellaneous code cleaning

- add <stdarg.h> or <varargs.h> in snprintf.h
  and remove those inclusions in the other #ifdef NEED_SNPRINTF_H codes

- remove the check of multiple inclusions in source (.c)  code
  (there is a bit loss of _cpp_ performance, but I prefer the gain of
   code reading and maintenance; and nowadays, disk caches and VM are
   correctly optimized ;-).

- protect all (well almost) header files against multiple inclusions

- add header (i.e. GPL license) in some include files

- reorganize a bit the way header files are included:

  First:
  #include <system_include_files>
  #include <external_package_include_files (e.g. gtk, glib etc.)>
  Then
  #include "ethereal_include_files"

  with the correct HAVE_XXX or NEED_XXX protections.

- add some HAVE_XXX checks before including some system header files

- add the same HAVE_XXX in wiretap as in ethereal

Please forgive me, if I break something (I've only compiled and regression
tested on Linux).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@2254 f5534014-38df-0310-8fa8-9805f1628bb7