Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Convert usr.sbin to LIBADD
Reduce overlinking

Use the new bsd.snmpmod.mk makefile to build the modules. The -I pointers
into the contrib directory are still necessary for some of the Makefiles,
because the C-sources there use non-canonical includes ("" includes) to get
at the header files.

Rename the MIBS makefile variable to BMIBS so that it does not conflict
with the environment variable MIBS that is used by net-snmp.

Create the OID and tree files while building the modules and the daemon
instead of creating them by hand and storing them in the CVS tree. Add
gensnmptree to the bootstrap tools (it is used to generated these files).
This simplifies the update procedure.

Submitted by: ru

Clean up makefiles.

Reviewed by: harti

The snmp_netgraph module depends on libnetgraph. So add a dependency and
add libnetgraph to the list of prebuilt libraries in the main Makefile.

Reviewed by: ru

Fix the build by adjusting the include directives. The problem was
that the sources use "..." includes to get at include files that
later on reside in an include sub-directory.

Add build infrastructure for libbsnmp and the modules. Add the
netgraph module for the SNMP daemon.