El compiler generates calls to println, print, exit_program,
http_set_handler, http_serve, http_set_handler_v2, and http_serve_v2
as el_val_t-returning functions. The runtime declared them void,
causing conflicting-type errors when el-install.c was compiled.
Change all seven to return el_val_t (side-effect functions return 0).
Also update el_runtime.h declarations to match.