[Rd] Dynamic help pages

[Duncan Murdoch]

I've just committed code that allows R to be built with support for 
dynamic help pages using \Sexpr in Rd files.  Define USE_NEW_HELP and 
the R function tools:::.convertRdfiles will be used instead of the Perl 
RdConv script, supporting \Sexpr and using the new R converters.

Currently only text help has the macros evaluated when the user asks for 
help; the others are rendered at install time.  Hopefully HTML rendering 
will also be in place before the 2.10.x release.  I can't see how CHM 
and Latex help pages would ever be dynamic. Build time evaluation of 
\Sexpr macros is not yet supported.

The only man page currently containing \Sexpr is ?tools::Rd2HTML.

Duncan Murdoch