Tweak the knots chunked output port

To try and reduce the number of write syscalls.

knots/web-server.scm

@@ -63,6 +63,14 @@
     (bind sock family addr port)
     sock))
 
+(define crlf-bv
+  (string->utf8 "\r\n"))
+
+(define (chunked-output-port-overhead-bytes write-size)
+  (+ (string-length (number->string write-size 16))
+     (bytevector-length crlf-bv)
+     (bytevector-length crlf-bv)))
+
 (define* (make-chunked-output-port/knots port #:key (keep-alive? #f)
                                          (buffering 1200))
   "Returns a new port which translates non-encoded data into a HTTP
@@ -74,10 +82,12 @@ when done, as it will output the remaining data, and encode the final
 zero chunk. When the port is closed it will also close PORT, unless
 KEEP-ALIVE? is true."
   (define (write! bv start count)
-    (put-string port (number->string count 16))
-    (put-string port "\r\n")
+    (let ((len-string
+           (number->string count 16)))
+      (put-string port len-string))
+    (put-bytevector port crlf-bv 0 2)
     (put-bytevector port bv start count)
-    (put-string port "\r\n")
+    (put-bytevector port crlf-bv 0 2)
     (force-output port)
     count)
 
@@ -401,7 +411,10 @@ on the procedure being called at any particular time."
                                  (make-chunked-output-port/knots
                                   client
                                   #:keep-alive? #t
-                                  #:buffering buffer-size))))
+                                  #:buffering
+                                  (- buffer-size
+                                     (chunked-output-port-overhead-bytes
+                                      buffer-size))))))
                        (set-port-encoding! body-port charset)
                        (let ((body-written?
                               (with-exception-handler