Initial support for HTML Help. All the ad-hoc help-file finding code

and various calls to WinHelp() have been centralised into a new file
winhelp.c, which in turn has been modified to detect a .CHM file as
well as .HLP and select between them as appropriate. It explicitly
tries to load HHCTRL.OCX and use GetProcAddress, meaning that it
_should_ still work correctly on pre-HTML-Help platforms, falling
gracefully back to WinHelp, but although I tested this by
temporarily renaming my own HHCTRL.OCX I haven't yet been able to
test it on a real HTML-Help-free platform.

Also in this checkin: a new .but file and docs makefile changes to
make it convenient to build the sources for a .CHM. As yet, owing to
limitations of Halibut's CHM support, I'm not able to write a .CHM
directly, more's the pity.

[originally from svn r7000]

doc/Makefile

@@ -49,6 +49,10 @@ index.html: $(INPUTS)
 putty.info: $(INPUTS)
 	$(HALIBUT) --info $(INPUTS)
 
+chm: putty.hhp
+putty.hhp: $(INPUTS) chm.but
+	$(HALIBUT) --html $(INPUTS) chm.but
+
 MKMAN = $(HALIBUT) --man=$@ mancfg.but $<
 MANPAGES = putty.1 puttygen.1 plink.1 pscp.1 psftp.1 puttytel.1 pterm.1
 man: $(MANPAGES)
@@ -62,4 +66,4 @@ puttytel.1: man-ptel.but mancfg.but; $(MKMAN)
 pterm.1: man-pter.but mancfg.but; $(MKMAN)
 
 clean:
-	rm -f *.html *.txt *.hlp *.cnt *.1 *.info vstr.but
+	rm -f *.html *.txt *.hlp *.cnt *.1 *.info vstr.but *.hh[pck] *.chm