bug#18127: add-to-load-path adds a path twice

[Alex Kost <alezost@gmail.com>]

Hello, I noticed that ‘add-to-load-path’ adds a path 2 times when it is
called in Guile REPL.

taylanub from #guile explained that it may happen because in the REPL
the compilation and execution happen in the same session and
“‘add-to-load-path’ takes care of modifying the path both at
compile-time and at run-time.” (from manual).

Here is the session for illustrating the bug (if it's a bug) :

$ guile
GNU Guile 2.0.11
Copyright (C) 1995-2014 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> %load-path
$1 = ( "/usr/share/guile/2.0" "/usr/share/guile/site/2.0" "/usr/share/guile/site" "/usr/share/guile")
scheme@(guile-user)> (add-to-load-path "foo")
scheme@(guile-user)> %load-path
$2 = ("foo" "foo" "/usr/share/guile/2.0" "/usr/share/guile/site/2.0" "/usr/share/guile/site" "/usr/share/guile")
scheme@(guile-user)> ,q


Also I have a question: Why there is no ‘add-to-load-compiled-path’?  I
think such function may also be useful.

Thanks,
Alex