Use va_copy() on all platforms; fixed a dangerous low-level bug (#160)

Use va_copy() on all platforms; fixed a dangerous low-level bug (#160)

To improve cross-compilation support and to simplify code, rely on C++11
cstdarg header instead of ./configure-time va_copy() detection.

Using ./configure-time detection for va_copy() is dangerous because when
it does not work (e.g., during a poorly configured cross-compilation
attempt), Squid may crash if va_copy() was needed but was not detected.

See also: Bug 4821 and bug 753.

Also found and fixed a low-level bug: StoreEntry::vappendf() was not
using va_copy() because store.cc lacked VA_COPY #defines. The affected
code (900+ callers!) is used for cache manager responses and Gopher
gateway response compilation. If any of those calls required a buffer
larger than 4KB, the lack of those va_copy() calls could lead to crashes
and/or data corruption issues on platforms where va_copy() is required.