Minor cleanup in _lwan_append_str_escaped_to_strbuf()
Escape characters according to OWASP XSS rule #1

https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet
Minor cleanups in lwan-socket.c, to move things closer to where they're used
Minor cleanups in lwan-template.c