parser: Fix backquote support in here-document EOF mark

parser: Fix backquote support in here-document EOF mark

Upstream commit:

    Author: Herbert Xu <herbert@gondor.apana.org.au>
    Date:   Thu Mar 15 18:27:30 2018 +0800
    parser: Fix backquote support in here-document EOF mark

    Currently using backquotes in a here-document EOF mark is broken
    because dash tries to do command substitution on it.  This patch
    fixes it by checking whether we're looking for an EOF mark during
    tokenisation.

    Reported-by: Harald van Dijk <harald@gigawatt.nl>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
With added fix for quoted-ness of the EOF mark.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>