Sync src with amalgamated versions

Now src contains exactly what's embedded in mongoose.c and .h, nothing more.

Added `tools/amalgam.sh` to aamlgamate both files at once.

There are no functional changes to mongoose.c, .h in this PR, only slight filename changes.

tools/amalgam.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -x -e
+
+tools/amalgam.py --prefix=MG --license=LICENSE $(cat src/mongoose.h.manifest) > mongoose.h
+tools/amalgam.py --prefix=MG --public-header=mongoose.h --license=LICENSE $(cat src/mongoose.c.manifest) > mongoose.c