[tex-live] w32: default multi-user

[Siep Kroonenberg]

A new Windows wrapper for TeX Live Manager now triggers a UAC prompt
if it needs admin permissions and does not have them. On XP it bails
out with an explanation in such cases.

With this change, it is reasonable to make a multiuser install the
default when the installer is run with admin privileges.

Note that it is still possible to mess up a multiuser install by
running tlmgr from the command-line as a non-privileged user.

-- 
Siep Kroonenberg