refactor: tool/walbctl.c.
tool: update Makefile.
wlog: remove check_userland.h.
wlog: util.hpp: remove deprecated definitions.
wlog: util.hpp: add LibcError.
wlog: aio_util.hpp: improve error handling.
wlog: update Makefile.
wlog: aio_util.hpp: a bit fix.
wlog: util.hpp: remove unused functions/classes.
wlog: util.hpp: use LibcError instead std::runtime_error for errors of libc functions.