ash: fix EXEXEC status clobbering

ash: fix EXEXEC status clobbering

evalcommand always clobbers the exit status in case of an EXEXEC
which means that exec always fails with exit status 2 regardless
of what it actually returns.

This patch adds the missing check for EXEXEC so that the correct
exit status is preserved.  It causes the test ash-misc/exec.tests
to succeed.

Based on commit 7f68426 in dash git, by Herbert Xu.

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