authentication: use hidden form instead of referer

This also gives us some CSRF protection. Note that we make use of the
hmac to protect the redirect value.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgitrc.5.txt

@@ -662,7 +662,8 @@ auth filter::
 	the http cookie and return a 0 if it is invalid or 1 if it is invalid,
 	in the exit code / close function. If the filter action is
 	"authenticate-post", this filter receives POST'd parameters on
-	standard input, and should write to output one or more "Set-Cookie"
+	standard input, and should write a complete CGI request, preferably
+	with a 302 redirect, and write to output one or more "Set-Cookie"
 	HTTP headers, each followed by a newline.
 
 	Please see `filters/simple-authentication.lua` for a clear example