Fixes for 0.8.1 release

I intend these three commits, applied on top of the v0.8.0 tag, to be the 0.8.1 release.

The plan is:

+ A new checkblock rule, in effect until 15 May, that ensure only blocks compatible with old releases are accepted into the main chain (only blocks that touch 4,500 or fewer distinct txids are allowed).

+ A limit of 500k to blocks created, also in effect until 15 May.

Alerts will be sent to pre-0.8 releases over the next two months, telling people to either upgrade or create a DB_CONFIG file so they can handle large blocks. After May 15'th, blocks up to 1MB large will be allowed again.

Tested by syncing the entire main chain and the testnet chains. A unit test is included, but it only runs if you download the 900K forking block (see the comment in src/test/checkblock_tests.cpp ).