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/main.cpp

@@ -1755,7 +1755,8 @@ void static UpdateTip(CBlockIndex *pindexNew) {
     cvBlockChange.notify_all();
 
     // Check the version of the last 100 blocks to see if we need to upgrade:
-    if (!IsInitialBlockDownload())
+    static bool fWarned = false;
+    if (!IsInitialBlockDownload() && !fWarned)
     {
         int nUpgraded = 0;
         const CBlockIndex* pindex = chainActive.Tip();
@@ -1768,8 +1769,12 @@ void static UpdateTip(CBlockIndex *pindexNew) {
         if (nUpgraded > 0)
             LogPrintf("SetBestChain: %d of last 100 blocks above version %d\n", nUpgraded, (int)CBlock::CURRENT_VERSION);
         if (nUpgraded > 100/2)
+        {
             // strMiscWarning is read by GetWarnings(), called by Qt and the JSON-RPC code to warn the user:
             strMiscWarning = _("Warning: This version is obsolete, upgrade required!");
+            CAlert::Notify(strMiscWarning, true);
+            fWarned = true;
+        }
     }
 }