munkiimport: Add argument to specify package icon

This patch adds a command line option to munkiimport for specifying an icon file for the package.  The specified icon will be renamed and copied to the REPO_PATH/icons folder as appropriate.  It also consolidates some repeated icon import code into a function.

On line 995 there is an `if False:` statement.  I can't tell if this was just a stub for future improvement or a fix to prevent an undesirable effect of the icon import code.

I believe this would be one method of resolving issue #371.  If you would prefer to have an interactive prompt for a manual icon path in the event that an icon is not automatically found just let me know and I'll recode.  I was attempting to minimize admin interaction.