Header limit of 32 KiB, upstream URL checks, build and run scripts

The request line and headers are now refused above 32 KiB exactly: Go's
server reads 4 KiB past MaxHeaderBytes, so that is set to 28 KiB, and
the tests try exactly 32 KiB and one byte more. SWWAF_UPSTREAM_URL now
needs a host, and a given port must be from 1 to 65535. make build and
make run call script/build and script/run.

Model: opus-5-5

internal/proxy/proxy.go

@@ -15,11 +15,14 @@ import (
 
 // The request line and headers a client may send, and how long a
 // kept-open client connection may wait for its next request, are fixed
-// rather than settings. The idle time is longer than the 90 seconds after
-// which traefik closes a connection it is not using, so traefik never
-// sends a request on a connection smallwebwaf is closing.
+// rather than settings. The limit on the request line and headers is
+// 32 KiB, but Go's server reads 4 KiB past its MaxHeaderBytes before it
+// refuses, so MaxHeaderBytes is set 4 KiB lower. The idle time is longer
+// than the 90 seconds after which traefik closes a connection it is not
+// using, so traefik never sends a request on a connection smallwebwaf is
+// closing.
 const (
-	requestHeaderMaxBytes = 32 << 10
+	requestHeaderMaxBytes = 32<<10 - 4<<10
 	clientIdleTimeout     = 120 * time.Second
 )