[log] use UUID instead of MD5(time) when creating log files
[doc] update manpage paragraph about --log
[*] update references to luaotfload-override/luaotfload-log
[log,*] move logs -> luaotfload.log

Next step toward https://github.com/lualatex/luaotfload/issues/169

Signed-off-by: Philipp Gesang <phg42.2a@gmail.com>