Detect Window's style slashes in mg_upload file names

MKCOL and DELETE

support serving pre-gzipped files

Added bounds checking for listening_ports.

support serving pre-gzipped files transparently provided the user agent accepts gzip content-encoding. The feature breaks if the user agent makes a range request within a gzipped file, and returns 501 in this case

Needs to be a valid TCP port number, and not less than 1 or greater than 65535

Securely close SSL connection by standard SSL_shutdown

For SSL in close_connection, current implementation use close_socket_gracefully to close socket firstly, then SSL_free. In my
testing, this approach may cause some pending write data missing. I'd suggest use standard SSL_shutdown in SSL conection. It
need call twice according to openSSL manual - one call sends the close_notify alert and one call receives the response from
the peer