Centralized inclusion of stdarg.h. This avoids compilation errors in

rtldebug mode on some platforms.

Rev: src/global.h:1.113
Rev: src/module_support.h:1.19
Rev: src/pike_error.h:1.50
Rev: src/port.h:1.65
Rev: src/program.h:1.260
Rev: src/stralloc.h:1.111