#2628 Disabled GET requests if --disable-set-support is used

[scai]

Commit #01c930e disabled various GET request handlings if --disable-set-support is used.

On various places HANDLER_CAN_RWRITE had been completely disabled if NETSNMP_DISABLE_SET_SUPPORT is defined. This is wrong because HANDLER_CAN_RWRITE enables both read and write support, removing it also disables read support. If only write support shall be disabled then HANDLER_CAN_RWRITE has to be replaced with HANDLER_CAN_RONLY.

Commit #be35e24e already fixed some of these issues but left many unfixed. I've attached a patch to (hopefully) fix all of these issues.

[Bill Fenner]

Thanks! Committed in 9c66a12ad9c50ae52a89b1fbe8600d1813b6fc2b on V5-7-patches.