Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make NETGRAPH_DEBUG a kernel option, so that it can't be turned off
without hacking source.

In collaboration with: ru, julian

Don't export symbols, all netgraph modules appear to have
proper module dependencies.

While here, removed stray -Wall from CFLAGS.

Turn on unconditional symbol export for modules whose API is
not clear enough, will require additional analysis, or will
require some input from their maintainers.

Whistle's Netgraph link-layer (sometimes more) networking infrastructure.
Been in production for 3 years now. Gives Instant Frame relay to if_sr
and if_ar drivers, and PPPOE support soon. See:
ftp://ftp.whistle.com/pub/archie/netgraph/index.html
for on-line manual pages.

Reviewed by: Doug Rabson (dfr@freebsd.org)
Obtained from: Whistle CVS tree