ash: [BUILTIN] Use EXEXIT in place of EXEXEC

ash: [BUILTIN] Use EXEXIT in place of EXEXEC

Upstream commit:

    Date: Sun, 28 Nov 2010 20:44:37 +0800
    [BUILTIN] Use EXEXIT in place of EXEXEC

    The intended semantics of EXEXEC are identical to EXEXIT, so
    simplify by using EXEXIT directly.

    Functional change: in edge cases (exec within a trap handler),
    this causes the exit status from exec not to be clobbered.
    For example, without this patch:

     $ sh -c 'trap "exec nonexistent" EXIT'; echo $?
     exec: 1: nonexistent: not found
     0

    And with it:

     $ sh -c 'trap "exec nonexistent" EXIT'; echo $?
     exec: 1: nonexistent: not found
     127

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