Add more documentation to redbean

This change also improves the unix module, adding a reboot() system call
for fun and profit, fixing the execve() api, and a printimage release.

tool/net/demo/unix2.lua

@@ -0,0 +1,26 @@
+-- example of how to run the ls command
+-- and pipe its output to the http user
+local unix = require "unix"
+ls = unix.commandv("ls")
+reader, writer = unix.pipe()
+if unix.fork() == 0 then
+   unix.close(1)
+   unix.dup(writer)
+   unix.close(writer)
+   unix.close(reader)
+   unix.execve(ls, {ls, "-Shal"})
+   unix.exit(127)
+else
+   unix.close(writer)
+   SetHeader('Content-Type', 'text/plain')
+   while true do
+      data = unix.read(reader)
+      if data ~= "" then
+         Write(data)
+      else
+         break
+      end
+   end
+   unix.close(reader)
+   unix.wait(-1)
+end