SL-10077: Save correct uninstall directory and (un)install mode.

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.