ash: exec: Return 126 on most errors in shellexec

ash: exec: Return 126 on most errors in shellexec

Upstream commit:

    Date: Sat, 19 May 2018 02:39:37 +0800
    exec: Return 126 on most errors in shellexec

    Currently when shellexec fails on most errors the shell will exit
    with exit status 2.  This patch changes it to 126 in order to avoid
    ambiguities with the exit status from a successful exec.

    The errors that result in 127 has also been expanded to include
    ENOTDIR, ENAMETOOLONG and ELOOP.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
function                                             old     new   delta
shellexec                                            245     254      +9

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