Use scm_[to/from]_utf8_string instead of scm_[to/from]_locale_string as per guile recommendation

Notes:
- in some situations the original *locale_string variant is retained. This
  is when locale encoded strings are processed (mostly file names).
- the utf8 variants don't exist for guile 1.8. For that version these
  functions will be rededefined to call the locale variants.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23556 57a11ea4-9604-0410-9ed3-97b8803252fd
Bug 719481 - GnuCash report crashes with Guile2

This is actually a bug in swig.
This commit works around it by patching the swig
generated source files until swig itself has been fixed.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23557 57a11ea4-9604-0410-9ed3-97b8803252fd