Follow the Linux kernel's lead on "fallthrough"

The pseudo keyword 'fallthrough' is defined such that case statement
blocks must end with any of these keywords:
 * break;
 * fallthrough;
 * continue;
 * goto <label>;
 * return [expression];
 *
 *  gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes

The macro is defined either as

  __attribute__((__fallthrough__))

or as

  do {} while (0)  /* fallthrough */

not including the semicolon.

This change implements the Linux kernel style and updates several locations
where "/*fallthrough*/ and /* FALLTHROUGH */ were not previously replaced.

Externally imported code such as libedit, libtommath and sqlite are
restored to their unaltered state.

Change-Id: I69db8167b0d5884f55d96d72de3059a0235a1ba3

kadmin/rpc.c

@@ -972,8 +972,7 @@ process_stream(krb5_context contextp,
 	    INSIST(gctx.ctx == NULL);
 
 	    gctx.inprogress = 1;
-            /* FALLTHROUGH */
-	    fallthrough
+	    fallthrough;
 	case RPG_CONTINUE_INIT: {
 	    gss_name_t src_name = GSS_C_NO_NAME;
 	    krb5_data in;