fastcgi: Comply with RFC3875, ensure a leading slash on index rewrite

[francislavoie]

Closes #3568
Closes #3569

See https://tools.ietf.org/html/rfc3875#section-4.1.13 for SCRIPT_NAME requiring leading slash
See https://tools.ietf.org/html/rfc3875#section-4.1.15 for SERVER_PORT requiring omission if empty

[mholt]

LGTM, thank you very much again Francis!

[ttys3]

the latest implementation has a problem.

see

caddy/modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go

Line 301 in 07c7dc6

if reqPort != "" {

this makes it optional, not MUST be set like rfc said.

	reqHost, reqPort, err := net.SplitHostPort(r.Host)
	if err != nil {
		// whatever, just assume there was no port
		reqHost = r.Host
	}
	
	
	// compliance with the CGI specification requires that
	// SERVER_PORT should only exist if it's a valid numeric value.
	// Info: https://www.ietf.org/rfc/rfc3875 Page 18
	if reqPort != "" {
		env["SERVER_PORT"] = reqPort
	}

as https://tools.ietf.org/html/rfc3875#section-4.1.15

4.1.15.  SERVER_PORT

   The SERVER_PORT variable MUST be set to the TCP/IP port number on
   which this request is received from the client.  This value is used
   in the port part of the Script-URI.

      SERVER_PORT = server-port
      server-port = 1*digit

   Note that this variable MUST be set, even if the port is the default
   port for the scheme and could otherwise be omitted from a URI.

Note that this variable MUST be set

I fired up a PR #4572