ash: Return without arguments in a trap should use status outside traps

ash: Return without arguments in a trap should use status outside traps

Fixes exitcode_trap4.tests.
Upstream commit:

    Date: Mon, 6 Oct 2014 21:51:26 +0800
    Return without arguments in a trap should use status outside traps

    POSIX now requires that return without arguments in a trap should
    return the last command status prior to executing traps.  This
    patch implements this behaviour.

    Incidentally this also changes the behaviour of return without
    arguments in a loop conditional to use the last exit status in
    the body as opposed to the last command in the conditional when
    there is one.

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