Allow overriding path variables

Having them strictly hard-coded makes it difficult to write system tests
test in staged install scenarios.

src/bin/app.skel

@@ -8,12 +8,13 @@
     SATURNIN_APP_TAGLINE="__MKIT_PROJ_TAGLINE__"
     SATURNIN_APP_VERSION="__MKIT_PROJ_VERSION__"
     SATURNIN_APP_GIT_HASH="__MKIT_PROJ_GIT_LASTHASH__"
-    SATURNIN_CACHE_HOME="__SATURNIN_CACHE_HOME__"
-    SATURNIN_CONFIG_HOME="__SATURNIN_CONFIG_HOME__"
-    SATURNIN_DATA_HOME="__SATURNIN_DATA_HOME__"
-    SATURNIN_LIBEXEC="__SATURNIN_LIBEXEC__"
+    SATURNIN_CACHE_HOME=${SATURNIN_CACHE_HOME:-"__SATURNIN_CACHE_HOME__"}
+    SATURNIN_CONFIG_HOME=${SATURNIN_CONFIG_HOME:-"__SATURNIN_CONFIG_HOME__"}
+    SATURNIN_DATA_HOME=${SATURNIN_DATA_HOME:-"__SATURNIN_DATA_HOME__"}
+    SATURNIN_META_HELP=${SATURNIN_META_HELP:-"+__SATURNIN_META_HELP__"}
+    SATURNIN_LIBEXEC=${SATURNIN_LIBEXEC:-"__SATURNIN_LIBEXEC__"}
     SATURNIN_LIBEXEC_PREFIX="__MKIT_PROJ_PKGNAME__-"
-    SHELLFU_PATH="__SATURNIN_SHELLFU_DIR__"
+    SHELLFU_PATH=${SHELLFU_PATH:-"__SATURNIN_SHELLFU_DIR__"}
     PRETTY_USAGE="subcommand"
     PRETTY_DEBUG_EXCLUDE="$PRETTY_DEBUG_EXCLUDE,inigrep,saturnin"
 }