Guile startup cleaned up a bit

[Jim Blandy <jimb@red-bean.com>]

This is a change I think a lot of people have asked for.  I think it
makes Guile initialization a bit more rational.  

It does mean that you can't avoid the delay of loading boot-9.scm by
not calling scm_compile_shell_switches.  But I think it's better for
folks to edit things out of boot-9.scm that they don't care about than
to not load boot-9.scm at all, and leave Guile half-initialized.

If folks think this is wrong, please let me know.  At this point, some
of you have done a lot more work as a user of Guile than I have, so
you may well understand the issuse better than I do.


* Changes to the gh_ interface

** The gh_enter function now takes care of loading the Guile startup files.
gh_enter works by calling scm_boot_guile; see the remarks below.


* Changes to the scm_ interface

** The function scm_boot_guile now takes care of loading the startup files.

Guile's primary initialization function, scm_boot_guile, now takes
care of loading `boot-9.scm', in the `ice-9' module, to initialize
Guile, define the module system, and put together some standard
bindings.  It also loads `init.scm', which is intended to hold
site-specific initialization code.

Since Guile cannot operate properly until boot-9.scm is loaded, there
is no reason to separate loading boot-9.scm from Guile's other
initialization processes.

This job used to be done by scm_compile_shell_switches, which didn't
make much sense; in particular, it meant that people using Guile for
non-shell-like applications had to jump through hoops to get Guile
initialized properly.

** The function scm_compile_shell_switches no longer loads the startup files.
Now, Guile always loads the startup files, whenever it is initialized;
see the notes above for scm_boot_guile and scm_load_startup_files.

** Function: scm_load_startup_files
This new function takes care of loading Guile's initialization file
(`boot-9.scm'), and the site initialization file, `init.scm'.  Since
this is always called by the Guile initialization process, it's
probably not too useful to call this yourself, but it's there anyway.