feature: made the special $LIBxxx_PATH variable more general in the "--- stap" section.
doc: documented the section "--- init".
bugfix: when user specifies "Host" and "Connection" in "--- more_headers", then we should not add our own Host request headers. Also made the default value of the "Connection" request header to be "close" instead of "Close".
Test::Nginx::Socket: export new perl functions "add_cleanup_handler", "add_block_preprocessor", and "bail_out".