(cherry picked from commit 5fcb811a)

1. When SL window closes it doesn't mean that process died so gave it a bit more time to terminate
2. Added a retry logic in case first run happened too early for some reason
3. Replaced ok/cancel on locked file with abort/retry/ignorei

This reverts commit 681298dd, reversing
changes made to 323f41f4.

The installer is (once again) running with Admin privilege elevation even for
a Standard Windows user, so it can't determine the underlying Standard user.
Therefore it can't clean up any per-user shortcuts left over for that Standard
user. Moved the kludge to SLVersionChecker.

Curiously, the value of $INSTDIR is correct before MULTIUSER_UNINIT, but is
made incorrect by MULTIUSER_UNINIT. Save and restore the correct value.

Saving $MultiUser.InstallMode in the registry (by setting relevant macros
examined by MultiUser.nsh) is susceptible to overwriting if the user installs
multiple viewers with the same channel name. Instead, write an InstallMode.txt
in the install directory, and read it back on uninstall. Of course, add it to
the files to be deleted on uninstall.

Back out misguided attempt to overwrite $PROGRAMFILES with $PROGRAMFILES64.