1. Fix a crash, which happened when cdtext_get() was called for an emtp=
y
(i.e. NULL) field.
2. Add another function cdtext_get_const(), which returns a const point=
er
and avoids too much strcpying (apps may want only to TEST if a
field is present or have their own string management routines).
configure.ac: remove AIX driver for now - it doesn't really work
remove some (but not all) of the valgrind errors in cd-text
Some stylistic things, doxygen comment improvements typos, etc.
