MIfying stdarg.h and dropping varargs.h

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
http://www.netbsd.org/~joerg/stdarg.diff is a rather large patch to get
the following changes in:

- Drop varargs.h support. Pre-ANSI support is pretty pointless nowadays
  and it hasn't been supported by modern compilers for a long time.

- Merge all the MD stdarg.h implementations into sys/stdarg.h.
  There are two major cases here:
  - lint, which is stupid and gets some fake implementation of the
  builtins.
  - Real compiler, which are supposed to provide the GCC 4.5 builtins.
    Compiler not visible as GCC 4.5 map __builtin_va_start to
    __builtin_stdarg_start.

- Fix the _BSD_VA_LIST_ usage to actually define va_list in various
  places as required by POSIX.

- Include sys/stdarg.h from sys/systm.h, it's used enough to simplify
  the kernel sources that way.

- Stop abusing _BSD_VA_LIST_, just go with va_list directly, defining it
  as needed or including stdarg.h.

I have done release builds for a bunch of different architectures, but
not exhaustively, so there may be a build error here and there left.

Joerg