shared: add new "vpick" concept for ".v/" directories that contain versioned resources

shared: add new "vpick" concept for ".v/" directories that contain versioned resources

This adds a new concept for handling paths. At appropriate places, if a
path such as /foo/bar/baz.v/ is specified, we'll
automatically enumerate all entries in /foo/bar/baz.v/baz* and then
do a version sort and pick the newest file.

A slightly more complex syntax is available, too:

/foo/bar/baz.v/quux___waldo

if that's used, then we'll look for all files matching
/foo/bar/baz.v/quux*waldo, and split out the middle, and version sort
it, and pick the nwest.

The ___ wildcard indicates both a version string, and if needed an
architecture ID, in case per-arch entries shall be supported.

This is a very simple way to maintain versioned resources in a dir, and
make systemd's components automatically pick the newest. Example:

    /srv/myimages.v/foobar_1.32.65_x86-64.raw
    /srv/myimages.v/foobar_1.33.45_x86-64.raw
    /srv/myimages.v/foobar_1.31.5_x86-64.raw
    /srv/myimages.v/foobar_1.31.5_arm64.raw

If now nspawn is invoked like this:

    systemd-nspawn --image=/srv/myimages.v/foobar___.raw

Then it will automatically pick
/srv/myimages.v/foobar_1.33.45_x86-64.raw as the version to boot on
x86-64, and /srv/myimages.v/foobar_1.31.5_arm64.raw on arm64.

This commit only adds the basic implementation for picking files from a
dir, but no hook-up anywhere.