path: Skip directories when finalising $PATH search

Imagine $PATH /a:/b. There is an echo command at /b/echo. Under this
configuration, this works fine:

    % systemd-run --user --scope echo .
    Running scope as unit: run-rfe98e0574b424d63a641644af511ff30.scope
    .

However, if I do `mkdir /a/echo`, this happens:

    % systemd-run --user --scope echo .
    Running scope as unit: run-rcbe9369537ed47f282ee12ce9f692046.scope
    Failed to execute: Permission denied

We check whether the resulting file is executable for the performing
user, but of course, most directories are anyway, since that's needed to
list within it. As such, another is_dir() check is needed prior to
considering the search result final.

Another approach might be to check S_ISREG, but there may be more gnarly
edge cases there than just eliminating this obviously pathological
example, so let's just do this for now.

(cherry picked from commit 8b5cb69b)