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

configure.ac

@@ -509,12 +509,12 @@ dnl Deal with switch FALLTHROUGH
 AH_TOP([
 #if defined(__GNUC__)
 #if __GNUC__ >= 7
-# define fallthrough __attribute__((fallthrough));
+# define fallthrough __attribute__((fallthrough))
 #else
-# define fallthrough
+# define fallthrough do {} while (0) /* fallthrough */
 #endif
 #else
-# define fallthrough
+# define fallthrough  do {} while (0) /* fallthrough */
 #endif
 ])