Add ProgramMaxPayloadSize() API to Redbean

This way we don't have to use the flag and can configure it using
.init.lua instead.

tool/net/help.txt

@@ -1081,6 +1081,20 @@ FUNCTIONS
           operating system to choose a port, which may be revealed later on
           by GetServerAddr or the -z flag to stdout.
 
+  ProgramMaxPayloadSize(int64)
+          Sets the maximum HTTP message payload size in bytes. The
+          default is very conservatively set to 65536 so this is
+          something many people will want to increase. This limit is
+          enforced at the transport layer, before any Lua code is
+          called, because right now redbean stores and forewards
+          messages. (Use the UNIX API for raw socket streaming.) Setting
+          this to a very high value can be useful if you're less
+          concerned about rogue clients and would rather have your Lua
+          code be granted more control to bounce unreasonable messages.
+          If a value less than 1450 is supplied, it'll automatically be
+          increased to 1450, since that's the size of ethernet frames.
+          This function can only be called from .init.lua.
+
   ProgramPrivateKey(pem:str)
           Same as the -K flag if called from .init.lua, e.g.
           ProgramPrivateKey(LoadAsset("/.sign.key")) for zip loading or