[AGENT++] Patches / improvements to SNMP++, AGENT++, AgentX++

[Jeremy Nimmer]

Hello Frank, Jochen, and all,

Please find attached a tarball of 18 patches to SNMP++, AGENT++, and
AgentX++.  They are grouped into different categories, hopefully to make
it easier to review and apply them to the main source tree.

Overview of the directories:

* const, lang, linkage:  These should be uncontroversial.  They fix
const-casting (gcc's -Wcast-qual), language mis-use problems, and
library linkage (the public symbols tables).

* pedantry, shadow, unused:  Cosmetic fixes, but important ones --
removing unused code and variables, removing some method<->variable
shadowing (gcc's -Wshadow), and initializing varibles before first
possible use.

Each patch has a short comment at top describing it's particular edit.

Thanks,
- Jeremy