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.

src/crypto/base64.cc

@@ -39,6 +39,7 @@
 static const char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
 static const unsigned char reverse[] = {
+  // clang-format off
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3e, 0xff, 0xff, 0xff, 0x3f,
@@ -55,6 +56,7 @@ static const unsigned char reverse[] = {
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
   0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  // clang-format on
 };
 
 /* Reverse maps from an ASCII char to a base64 sixbit value.  Returns > 0x3f on failure. */

src/crypto/ocb_internal.cc

@@ -24,6 +24,8 @@
 / Comments are welcome: Ted Krovetz <ted@krovetz.net> - Dedicated to Laurel K
 /------------------------------------------------------------------------- */
 
+// clang-format off
+
 #include "src/include/config.h"
 
 /* This module implements the ae.h interface for OpenSSL, Apple Common