maint: update to work with GCC7's -Werror=implicit-fallthrough=5

maint: update to work with GCC7's -Werror=implicit-fallthrough=5

* src/system.h (FALLTHROUGH): Define.
* src/cp.c (main): Use new FALLTHROUGH macro in place of comments.
* src/basename.c (main): Likewise.
* src/dircolors.c (append_quoted): Likewise.
* src/echo.c (main): Likewise.
* src/fold.c (main): Likewise.
* src/join.c (main): Likewise.
* src/kill.c (main): Likewise.
* src/ls.c (get_funky_string, gobble_file): Likewise.
* src/sort.c (parse_field_count, main): Likewise.
* src/stat.c (print_it): Likewise.
* src/tail.c (parse_obsolete_option): Likewise.
* src/test.c (posixtest): Likewise.
* src/wc.c (wc): Likewise.
* src/who.c (main): Likewise.