ash: eval: Replace with listsetvar with mklocal/setvareq

ash: eval: Replace with listsetvar with mklocal/setvareq

Upstream commit:

    Date: Sat, 19 May 2018 02:39:55 +0800
    eval: Replace with listsetvar with mklocal/setvareq

    This patch replaces listsetvar with mklocal/setvareq.  As we now
    determine special built-in status prior to variable assignment, we
    no longer have to do a second pass listsetvar.  Instead we will
    call setvareq directly instead of mklocal when necessary.

    In order to do this mklocal can now take a flag in order to mark
    a variable for export.

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