Add .clang-format and format C files (#2246)

Add `.clang-format` file that can be used directly by the clang
formatter or by the Microsoft C/C++ VSCode extension.
Style based on Microsoft default style guide, but update brace and case
indentation settings to match current files better (as well as Go code).

Run formatter on `init\init.c` and `vsockexec\vsockexec.c`.
Changes mostly with brace position, spacing, indentation, and pointer
alignment.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

.clang-format

@@ -0,0 +1,12 @@
+Language: Cpp
+BasedOnStyle: Microsoft
+BreakBeforeBraces: Attach
+PointerAlignment: Left
+AllowShortFunctionsOnASingleLine: All
+# match Go style
+IndentCaseLabels: false
+# don't break comments over line limit (needed for CodeQL exceptions)
+ReflowComments: false
+InsertNewlineAtEOF: true
+KeepEmptyLines:
+  AtEndOfFile: true