Rewrite HTTP server
Add alloca_poll_events() macro
Add 'strlen' argument to strn_fromprint()
Use size_t in Rhizome store functions
New header file "fdqueue.h"

So that "http_server.h" does not have to include "serval.h" which
creates a circular dependency.

Remove the __SERVALDNA__HTTP_SERVER_IMPLEMENTATION hack from
"http_server.h"
Improve str_to_int() et al

Always set *afterp to point to the first invalid character
even if conversion fails
Add str_tolower_inplace() function
Add alloca_strdup() macro