Switch to using sys/stdarg.h for va_list type and va_* builtins.
Make an attempt to insert the include in a sensible place. Where
style(9) was followed this is easy, where it was ignored, aim for the
first block of sys/*.h headers and don't get too fussy or try to fix
other style bugs.
Reviewed by: imp
Exp-run by: antoine (PR 286274)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1595
