Rename variable to reflect the path is being provided by saturnin

mkit/config.ini

     __SATURNIN_CONFIG_LOCAL__   = [roots:etc]
     __SATURNIN_LIBEXEC__        = [roots:libexec]
     __SATURNIN_SHARE__          = [roots:share]
-    __FFOO_DIR__                = [roots:share]/ffoo
+    __SATURNIN_FFOO_DIR__       = [roots:share]/ffoo
 
 [files:bin]
     src/bin/saturnin                        = saturnin