Trigger -alertnotify if network is upgrading without you

This adds a -regetest-only undocumented (for regression testing only)
command-line option -blockversion=N to set block.nVersion.

Adds to the "has the rest of the network upgraded to a
block.nVersion we don't understand" code so it calls
-alertnotify when 51 of the last 100 blocks are up-version.
But it only alerts once, not with every subsequent new, upversion
block.

And adds a forknotify.py regression test to make sure it works.

Tested using forknotify.py:

Before adding CAlert::Notify, get:
Assertion failed: -alertnotify did not warn of up-version blocks

Before adding code to only alert once:
Assertion failed: -alertnotify excessive warning of up-version blocks

After final code in this pull:
Tests successful

src/miner.cpp

@@ -83,6 +83,11 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         return NULL;
     CBlock *pblock = &pblocktemplate->block; // pointer for convenience
 
+    // -regtest only: allow overriding block.nVersion with
+    // -blockversion=N to test forking scenarios
+    if (Params().MineBlocksOnDemand())
+        pblock->nVersion = GetArg("-blockversion", pblock->nVersion);
+
     // Create coinbase tx
     CMutableTransaction txNew;
     txNew.vin.resize(1);