Rename some variables to avoid warnings on old gcc versions.

Rename some variables to avoid warnings on old gcc versions.

Fortunately, gcc-4 no longer warns on these. However, gcc-3.3 is still supported
for compiling paraslash, so rename "index" to "idx" and "conf" to "config" to get
rid of warnings of the form

wmadec_filter.c:1013: warning: declaration of `index' shadows a global declaration

Add documentation of ->free_config() while we're at it.