* lib/quotearg.c (FALLTHROUGH): New macro.
Use it whenever one switch case falls through into the next,
replacing "/* Fall through */" comments. This exposed one
instance of an unwarranted "fall through" comment: unwarranted
because it preceded a "goto" label not a case statement.
* lib/freopen-safer.c (freopen_safer): Likewise.
* lib/fts.c (leaf_optimization_applies): Likewise.
* lib/unistr/u8-uctomb-aux.c (u8_uctomb_aux): Likewise.
* tests/test-getopt_long.h (getopt_long_loop): Likewise.
* tests/test-tsearch.c (mangle_tree): Likewise. Also include
tests/macros.h for the definition.
* tests/test-argp.c (group1_parser): Likewise.
* tests/test-getopt.h (getopt_loop): Likewise.