[Rd] data frame subset patch, take 2

[Vladimir Dergachev]

Hi Robert,

Here is the second iteration of data frame subset patch.
It now passes make check on both 2.4.0 and 2.5.0 (svn as of a few days ago).
Same speedup as before.

Changes:

	* Introduced two new functions .subassign2 and .subassign that are 
complimentary to .subset2 and .subset.

	* Changed x[[j]]<- assignment to x<-.subassign2(x, j, ..) to fix the problem 
with the previous patch.

                               thank you !

                                    Vladimir Dergachev