Various code cleanup to avoid compiler, cppcheck, or clang warnings:

modules/debugging/mod_firehose.c: Make some internal functions static
                                  (to do: logs_cleanup() is unused)

modules/filters/mod_charset_lite.c: Remove dead assignments

modules/filters/mod_include.c: likewise

modules/metadata/mod_usertrack.c: likewise

modules/proxy/mod_proxy_ftp.c: likewise

modules/ssl/ssl_engine_pphrase.c: likewise

modules/proxy/mod_proxy_balancer.c: likewise;
                                    Remove NULL check that can never happen

modules/proxy/proxy_util.c: Axe NULL-check that can never happen and if it
                            would, it would just mask another bug

os/unix/unixd.c: likewise

modules/http/http_filters.c: Remove sub-condition that is always true

modules/lua/mod_lua.c: Add default cases to switch statements

modules/generators/mod_autoindex.c: Unsigned value can never be < 0

server/util_expr_eval.c: Fix compiler warnings with VC and on OS2



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