Remove the name, cmdline, and action parameters from imgdownload() and
imgdownload_string().  These functions now simply download and return
an image.

Add the function imgacquire(), which will interpret a "name or URI
string" parameter and return either an existing image or a newly
downloaded image.

Use imgacquire() to merge similar image-management commands that
currently differ only by whether they take the name of an existing
image or the URI of a new image to download.  For example, "chain" and
"imgexec" can now be merged.

Extend imgstat and imgfree commands to take an optional list of
images.

Remove the arbitrary restriction on the length of image names.

Signed-off-by: Michael Brown <mcb30@ipxe.org>