genl: Provide internal function to resolve name to id

Like genl_ops_resolve() but uses its own socket.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
genl: Update mt_id of cache ops when resolving genl id

The cache layer uses the message type array stored in the cache
ops to lookup which cache a message belongs to. Update to the
the message array with the resolved generic netlink id to make
it compatible with the caching API.

Allows to use nl_cache_refill() and others for generic netlink
based caches with dynamic generic netlink ids.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
cache: Improve debugging messages of cache operations

Signed-off-by: Thomas Graf <tgraf@suug.ch>
nl: Print file:line:func in debugging messages and provide --disable-debug to disable debugging

Compiling libnl with --disable-debug will result in the ignorance
of the 'NLDBG' environment variable.

Signed-off-by: Thomas Graf <tgraf@suug.ch>