MFC r256678,r256680,r260986,r272878,r286402:

MFC r256678,r256680,r260986,r272878,r286402:

r256678 (by syrinx):

Fix SNMP Error response PDUs and properly encode them when using v3 auth/encryption.

r256680 (by syrinx):

Fix the -Wconversion warnings produced when compiling the SNMP agent.

r260986 (by harti):

Fix a problem with OBJECT IDENTIFIER encoding: need to check the
second subid to be less than 40, not the first when the first
subid is 0 or 1.

r272878 (by syrinx):

Fix a bug in decoding string indexes in snmp_target(3), thus causing
bsnmpd(1) to not send v3 notifications properly; while here add two
missing return statements which could lead to abort() in case of a
rollback

r286402 (by araujo):

Fix variable 'old' is used uninitialized whenever '&&' condition is false.
Spotted by clang.

git-svn-id: svn://svn.freebsd.org/base/stable/10@301661 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f