Introduce VSIG which simply counts number of signals, which everybody
else can then look at.

If you use libvarnishapi and want your own signal handler, you must
increment these VSIG_{int|hup|term|usr1} counters yourself, for
libvarnishapi to work correctly.

The old VUT_Signal()/VUT_Signaled() API is retained and should be
deprecated in a future version.

Inspired by:	#2818