ash: optimize tryexec(): avoid one allocation

ash: optimize tryexec(): avoid one allocation

There was a bug in tryexec which bbox had fixed in 2003.
dash had a smaller fix in 2007. Copy it. It is smaller,
although it is also more quirky (requires argv[-1] to exist).

Upstream commit 1:

    Date: Mon, 15 Oct 2007 20:24:28 +0800
    [EXEC] Fixed execing of scripts with no hash-bang

    The function tryexec used the original name instead of the path found through
    PATH search.  This patch fixes that.

    Test case:

        trap 'rm -f $TMP' EXIT
        TMP=$(tempfile -s nosuchthing)

        cat <<- EOF > $TMP
                echo OK
        EOF
        chmod u+x $TMP

        cd /
        PATH=${TMP%/*} ${TMP##*/}

    Old result:

        /bin/sh: Can't open filelgY4Fanosuchthing

    New result:

        OK

Upstream commit 2:

    Date: Sun, 23 Dec 2007 11:02:26 +0800
    [EVAL] Fix bad pointer arithmetic in evalcommand

    dash dies on sparc with a SIGBUS due to an arithmetic error introduced
    with commit 03b4958, this patch fixes it.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
function                                             old     new   delta
evalcommand                                         1261    1264      +3
dotcmd                                               321     319      -2
tryexec                                              115      64     -51
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 3/-53)             Total: -50 bytes

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