ash: fix error during recursive processing of here document

ash: fix error during recursive processing of here document

Save the value of the checkkwd flag to prevent it being clobbered
during recursion.

Based on commit ec2c84d from git://git.kernel.org/pub/scm/utils/dash/dash.git
by Herbert Xu.

function                                             old     new   delta
readtoken                                            190     203     +13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 13/0)               Total: 13 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>