Support OnHttpRequest Lua callback

If your redbean `/.init.lua` file defines a global callable named
`OnHttpRequest` then redbean will delegate all serving control to
your function. You may then restore the default serving paths, by
calling the new `Route()`, `RouteHost()`, and `RoutePath()` APIs.

Closes #150

tool/net/demo/seekable.txt

@@ -0,0 +1,26 @@
+A
+B
+C
+D
+E
+F
+G
+H
+I
+J
+K
+L
+M
+N
+O
+P
+Q
+R
+S
+T
+U
+V
+W
+X
+Y
+Z