We rely on the include path to find many of these headers. It
especially doesn't make sense to use #include "foo.h" for headers
in the top-level include directory.
We now pass a pointer to the context where necessary. There are a
few cases where we need to request the context from sudoers via
sudoers_get_context() for the plugin API functions. If the plugin
API was able to pass around a closure pointer this would not be
necessary.
