ash: eval: Reap zombies after built-in commands and functions

ash: eval: Reap zombies after built-in commands and functions

Upstream commit:

    Date: Mon, 26 Mar 2018 23:55:50 +0800
    eval: Reap zombies after built-in commands and functions

    Currently dash does not reap dead children after built-in commands
    or functions.  This means that if you construct a loop consisting
    of solely built-in commands and functions, then zombies can hang
    around indefinitely.

    This patch fixes this by reaping when necessary after each built-in
    command and function.

    Reported-by: Denys Vlasenko <vda.linux@googlemail.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>