Batch of miscellaneous tweaks to the Windows installer:

Batch of miscellaneous tweaks to the Windows installer:
 - Now we've fixed `win-versioninfo', choose some sensible outcomes from
   the installer's comparisons of binary version numbers. Also, give the
   installer _itself_ a matching binary version.
   In particular, without this change, it would not have been possible
   to downgrade PuTTY -- it would have silently left the "newer" files in
   place. Now it will make some fuss, but permit it.
 - Also remove descriptions from shortcuts, on the grounds that the
   binaries have embedded descriptions now. (Although I've not checked
   whether those are actually visible in the Start Menu.)
 - At the request of various people (e.g., PJB), add flags so that if
   files are in use at the time the (un)installer is run, replacement is
   deferred to the next restart. (The user may be prompted to restart,
   which isn't ideal; see comments).
   This is supposed to make centrally-pushed silent upgrades more robust.
 - Note some limitations of the installer.

[originally from svn r6585]