Not loading boot-9.scm

[Maciej Stachowiak <mstachow@mit.edu>]

The new gh_enter behavior of not loading boot-9.scm is problematic for
apps that want boot-9.scm loaded but don't want to use
gh_repl. Indeed, I think loading or not loading of boot-9 should be
orthogonal to wether or not a repl is invoked. 

One of the most annoying problems is that it's somewhat complicated to
write an autoconf macro that detects wether or not gh_enter will load
boot-9 for you, and the scm_ice_9_already_loaded variable does not
accurately reflect wether it has been loaded already in older
versions.

I think a good thing to do would be to add a gh_enter_with_boot_9
which has the old behavior (this makes the autoconf test easy) or
better yet, restore gh_enter to the old behavior and add a
gh_enter_sans_boot_9. Another alternative would be to add an extra
argument to gh_enter, but this makes things hairier from the autoconf
standpoint. If people agree that one of these approaches is a good
idea, I can provide an appropriate patch.

Another note: without loading boot-9, guile fails to be R4RS
compliant. Perhaps there should be yet another option to load at least
r4rs.scm and define load to some suitable minimal thing.

 - Maciej Stachowiak