FastCGI: support for multibyte parameter name lengths.

FastCGI: support for multibyte parameter name lengths.

The name length of a configured parameter was truncated to one byte
before the per-parameter length field was written, so that a
fastcgi_param name of 128 bytes or longer was encoded with a wrong
length while the name itself was copied in full.  The resulting request
was malformed, and upstream servers responded by closing the connection.

While long names are barely used in practice, it is easy to support
them.  The length is now properly encoded up to the allowed length.