Don't use token for default name of SATURNIN_META_HELP

MKit (as of 0.0.25, and probably for long time in furure) ignores tokens
not defined in mkit.ini, so app developer has to either define it or
their users will see errors on every `app --help call`.

This way we can remove the need to define yet another token.

src/app.skel

     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_META_HELP=${SATURNIN_META_HELP:-"+__SATURNIN_SHARE__/help"}
     SATURNIN_LIBEXEC=${SATURNIN_LIBEXEC:-"__SATURNIN_LIBEXEC__"}
     SATURNIN_LIBEXEC_PREFIX="__MKIT_PROJ_PKGNAME__-"
     SHELLFU_PATH=${SHELLFU_PATH:-"__SATURNIN_SHELLFU_DIR__"}