replace '%zu' in fastcgi build

(this is just a WIP, not asking to really merge this)

This is a fix I still need, since libfcgi somehow doesn't work with `%zu`.

Examples:
```
$ curl -i http://localhost/cgi/download.fcgi
HTTP/1.1 200 OK
Date: Tue, 13 May 2014 15:50:03 GMT
Server: Apache/2.2.22 (Debian)
Content-Disposition: inline;filename="download.c"
Accept-Ranges: bytes
Content-Length: Connection: close
Vary: Accept-Encoding
Content-Type: text/plain
```
see the `Content-Length` header, fails to terminate.
With the fix:
```
$ curl -i http://localhost/cgi/download.fcgi
HTTP/1.1 200 OK
Date: Tue, 13 May 2014 15:53:43 GMT
Server: Apache/2.2.22 (Debian)
Content-Disposition: inline;filename="download.c"
Accept-Ranges: bytes
Connection: close
Content-Length: 1901
Vary: Accept-Encoding
Content-Type: text/plain
```

and in a `cookie.fcgi` body response:
```
Total 5 entries
Name6=Value232+6 (456+55=11 (Name1=Value2 (QSESSIONID=5d2b52357965d5adf3 (_Q_CONTENTTYPE=text/plain (
```
with the fix:
```
Total 5 entries
Name6=Value232+6 (11)
456+55=11 (3)
Name1=Value2 (7)
QSESSIONID=5d2b52357965d5adf3 (19)
_Q_CONTENTTYPE=text/plain (11)
```
Maybe there is a better way to fix this code, I don't know.
```