sh: Add case statement fallthrough (with ';&' instead of ';;').

sh: Add case statement fallthrough (with ';&' instead of ';;').

Replacing ;; with the new control operator ;& will cause the next list
to be executed as well without checking its pattern, continuing until
a list ends with ;; or until the end of the case statement. This is
like omitting "break" in a C "switch" statement.

The sequence ;& was formerly invalid.

This feature is proposed for the next POSIX issue in Austin Group
issue #449.

Obtained-from:   FreeBSD 223186