Add a clang-format file and prepare for clang-formatting

Create .clang-format to describe the current C++ style used in Mosh.

Mark one carefully-formatted array with `// clang-format off`. Also turn
off clang-format in src/crypto/ocb_internal.cc, since it was imported
almost wholesale from another project and is written in a style
different from the rest of Mosh.

.clang-format

@@ -0,0 +1,23 @@
+---
+Language: Cpp
+BasedOnStyle: Mozilla
+ColumnLimit: 116
+SpacesInParentheses: true
+AlwaysBreakAfterReturnType: None
+AlwaysBreakAfterDefinitionReturnType: None
+SpaceBeforeCpp11BracedList: true
+BreakBeforeBinaryOperators: All
+Cpp11BracedListStyle: true
+AllowShortBlocksOnASingleLine: Always
+BreakBeforeBraces: Custom
+BraceWrapping:
+  AfterClass: true
+  AfterControlStatement: Never
+  AfterFunction: true
+  AfterStruct: true
+  AfterEnum: true
+  SplitEmptyFunction: false
+  SplitEmptyRecord: false
+  SplitEmptyNamespace: false
+BreakConstructorInitializers: BeforeColon
+...