Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Do not add 'default:' cases inside a switch() on an enum like the
SNMP option argument. That way the compiler will give a warning in
case of a missing or an extra but unknown 'case:'.
Do the same for SNMP LEAF objects to be prepared once gensnmptree will
auto-generate enums for those too.
Add an abort() after the switch() instead of the 'default:' to catch
any errors. The nice side effect is that the compiler will correctly
track supposed to be 'uninitialized' variables with that.

When trying to set a value sanity check it before calling the OS
dependent API of snmp_bridge to tell whether it is an attempt to
set a bad value or a general error.

Suggested by: harti (first part)
Approved by: bz (mentor)

Instead of explicitly initializing variables to avoid compiler warnings,
add a default case to handle the situation when the variables are not
initialized.
Furthermore, abort() if the snmp agent passes an invalid option to the
bridge module. As the option (SET, GET, GETNEXT, COMMIT, ROLLBACK) is
determined by the snmp agent based on the operation requested by user,
this behaviour is unlikely to be abused as a source for a DoS, but if
ever hit will likely reveal a problem in the snmp agent or bridge
module.

Approved by: bz (mentor)

Bring in a SNMP module to support monitoring if_bridge(4) interfaces
via bsnmpd(1). The module implements IETF BRIDGE-MIB as defined in
RFC4188 and a private BEGEMOT-BRIDGE-MIB.

Sponsored by: Google Summer of Code 2006
Reviewed by: bz
Approved by: bz (mentor)