Add a helper API for ecscaping the value in audit log
messages

* src/util/virtaudit.h, src/util/virtaudit.c,
  src/libvirt_private.syms: Add virAuditEncode

* src/util/virtaudit.c (virAuditSend): one less ifdef, since gcc
does not care if an ATTRIBUTE_UNUSED var gets used in some paths.

Integrate with libaudit.so for auditing of important operations.
libvirtd gains a couple of config entries for auditing. By
default it will enable auditing, if its enabled on the host.
It can be configured to force exit if auditing is disabled
on the host. It will can also send audit messages via libvirt
internal logging API

Places requiring audit reporting can use the VIR_AUDIT
macro to report data. This is a no-op unless auditing is
enabled

* autobuild.sh, mingw32-libvirt.spec.in: Disable audit
  on mingw
* configure.ac: Add check for libaudit
* daemon/libvirtd.aug, daemon/libvirtd.conf,
  daemon/test_libvirtd.aug, daemon/libvirtd.c: Add config
  options to enable auditing
* include/libvirt/virterror.h, src/util/virterror.c: Add
  VIR_FROM_AUDIT source
* libvirt.spec.in: Enable audit
* src/util/virtaudit.h, src/util/virtaudit.c: Simple internal
  API for auditing messages