Add context help to a couple of message boxes. Unfortunately the ones

I wanted to get to -- "software caused connection abort" and friends --
are going to be more involved (probably requiring some cross-platform
notion of help contexts), and these ones hardly seem worth the effort.
Still, I've done it now.

Side-effect: Pageant now uses the same `hinst' and `hwnd' globals as
everything else. Tested basic functionality.

[originally from svn r5417]

doc/errors.but

@@ -130,9 +130,11 @@ Similarly, any error message starting with \q{Assertion failed} is a
 bug in PuTTY. Please report it to us, and include the exact text
 from the error message box.
 
-\H{errors-key-wrong-format} \q{Unable to use this private key file},
+\H{errors-cant-load-key} \q{Unable to use this private key file},
 \q{Couldn't load private key}, \q{Key is of wrong type}
 
+\cfg{winhelp-topic}{errors.cantloadkey}
+
 Various forms of this error are printed in the PuTTY window, or
 written to the PuTTY Event Log (see \k{using-eventlog}) when trying
 public-key authentication, or given by Pageant when trying to load a

doc/using.but

@@ -516,6 +516,8 @@ use the \c{-load} option (described in \k{using-cmdline-load}).
 
 \S{using-cleanup} \i\c{-cleanup}
 
+\cfg{winhelp-topic}{options.cleanup}
+
 If invoked with the \c{-cleanup} option, rather than running as
 normal, PuTTY will remove its \I{removing registry entries}registry
 entries and \I{random seed file} from the local machine (after