CAutoFile: Explicit Get() and remove unused methods

Also add documentation to some methods.

src/test/checkblock_tests.cpp

@@ -36,7 +36,7 @@ bool read_block(const std::string& filename, CBlock& block)
     fseek(fp, 8, SEEK_SET); // skip msgheader/size
 
     CAutoFile filein(fp, SER_DISK, CLIENT_VERSION);
-    if (!filein) return false;
+    if (filein.IsNull()) return false;
 
     filein >> block;