Clean up/robustify favicon.sh.

shell_scripts/favicon.sh
	-Replace the hard-coded, repetitive `convert` command with a
	for-loop that generates and then executes it.
Modify HardRetab() to restore cursor position.

vimrc
	-Modify HardRetab() to restore the cursor to the position that
	it occupied before the function was called.
	-Existing normal-command "``" wasn't doing it.