Add an .editorconfig; document git whitespace settings

Most of the projects I work on don't use tabs, and while authoring my
first patch I had to wrestle with my editor to not introduce
whitespace editors.

The `.editorconfig` file is supported by a large number of editors
out of the box, and many more with plugins.

As a first-time contributor, I can't say for certain these settings
are totally correct, but thus far git and my editor are satisfied
enough.

I considered adding `git config --local format.signOff true` but
wanted to respect the warning:

    format.signOff
        A boolean value which lets you enable the -s/--signoff
        option of format-patch by default.  Note: Adding the
        Signed-off-by: line to a patch should be a conscious act and
        means that you certify you have the rights to submit this
        work under the same open source license. Please see the
        SubmittingPatches document for further discussion.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Graham Christensen <graham@grahamc.com>
Closes #9892

.editorconfig

@@ -0,0 +1,10 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{c,h}]
+tab_width = 8
+indent_style = tab