Updating tlmgr from tlshell on windows

[Siep Kroonenberg]

On windows, an update of tlmgr from tlshell fails sometimes, e.g.
because windows complains that a to-be-updated file is in use.
So on windows tlshell informs about the command-line alternative and
asks for confirmation. Tomorrow's update should do the same for
`update all' if that includes tlmgr.

By the way: even on the command-line I encountered this problem
although there was no process in sight that could be using the
particular file. A reboot solved the problem. Well, windows remains
windows ...

And tlmgr.bat now tests for a gui parameter and bails out with a
reference to tlshell.

-- 
Siep Kroonenberg