build: update coding-style settings

Update .editorconfig, and add .clang-format.

Note that current code does _not_ follow that, but we'll slowly get there.

.editorconfig

@@ -2,6 +2,7 @@
 # editorconfig file (see EditorConfig.org), with some
 # lowest-denominator settings that should work for many editors.
 
+
 root                        = true # this is the top-level
 
 [*]
@@ -10,23 +11,20 @@ insert_final_newline        = true
 charset                     = utf-8
 trim_trailing_whitespace    = true
 
-# The "best" answer is "tabs-for-indentation; spaces for alignment", but in
-# practice that's hard to accomplish in many editors.
-#
-# So we use spaces instead, at least that looks consistent for all
+# The "best" answer is "tabs-for-indentation; spaces for alignment".
 
 [*.{cc,cpp,hh,hpp}]
-indent_style                = space
+indent_style                = tab
 indent_size                 = 8
 max_line_length             = 100
 
 [*.{c,h}]
-indent_style                = space
+indent_style                = tab
 indent_size                 = 8
 max_line_length             = 80
 
 [configure.ac]
-indent_style                = space
+indent_style                = tab
 indent_size                 = 4
 max_line_length             = 100