hc-install: support fetching of manually entered image URLs

Previously only a local file path was supported, now anything
supported by fetch(1) should work.
hc-install: add some options to image fetch(1) call

- use -1 to be explicit that we want a single file
- use -a to auto-retry to transient network problems
hc-install: gmirror clear can fail, so discard its exit status

just in case we might want to use set -e some day to catch all errors
hc-install: use correct variable for number of detected disks
hc-install: show disk sizes next to disk names in the menu

- using 'eval' to properly show disk name + size strings in dialog
- also reduced duplicated code a little bit
hc-install: save stderr to /tmp/install.log

also use set -x so that all commands get logged