Add an API for sending signals to arbitrary processes in the
guest OS. This is primarily useful for container based virt,
but can be used for machine virt too, if there is a suitable
guest agent,

* include/libvirt/libvirt.h.in: Add virDomainSendProcessSignal
  and virDomainProcessSignal enum
* src/driver.h: Driver entry point
* src/libvirt.c, src/libvirt_public.syms: Impl for new API
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>