build: Distinguish libnl version checks Use separate shell variable name prefixes for checks of separate libnl versions, as repeated invocations of PKG_CHECK_MODULES with the same prefix are not generally supposed to have an effect. This prevents bugs similar to https://fedorahosted.org/sssd/ticket/2388 Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Provide libnl3 support https://fedorahosted.org/sssd/ticket/812 Update the monitor code to be using the new libnl3 API. Changed configure option --with-libnl By default, it tries to build with libnl3, if not found, then with libnl1, if this isn't found either, build proceeds without libnl, just with warning. Specifing --with-libnl=<libnl3|libnl1|no> checks for the specific given version, if not found, configure ends with error.

Remove support for pre-1.1 netlink Netlink 1.0 and older is buggy and unreliable, occasionally causing tight-loops. We're no longer going to try to support it. https://fedorahosted.org/sssd/ticket/755

Use netlink to detect going online Integrates libnl to detect adding routes. When a route is added, the offline status of all back ends is reset. This patch adds no heuristics to detect whether back end went offline. Fixes: #456