When using g++ and enforcing strict compiler errors, this commit fixes the following:

mongoose.c: In function ‘char* mg_fgets(char*, size_t, file*, char**)’:
mongoose.c:2405:60: error: invalid conversion from ‘void*’ to ‘char*’ [-fpermissive]

mongoose.c: In function ‘int set_ports_option(mg_context*)’:
mongoose.c:4586:64: error: invalid conversion from ‘void*’ to ‘socket*’ [-fpermissive]

mongoose.c: In function ‘void* worker_thread(void*)’:
mongoose.c:5104:28: error: invalid conversion from ‘void*’ to ‘mg_context*’ [-fpermissive]

mongoose.c: In function ‘void* master_thread(void*)’:
mongoose.c:5220:28: error: invalid conversion from ‘void*’ to ‘mg_context*’ [-fpermissive]
mongoose.c:5235:58: error: invalid conversion from ‘void*’ to ‘pollfd*’ [-fpermissive]

mongoose.c:4680:23: error: ‘ERR_get_error’ was not declared in this scope
mongoose.c:4681:52: error: ‘ERR_error_string’ was not declared in this scope

Added check for document_root != NULL in convert_uri_to_file_name(). Added #define rmdir _rmdir for win32

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

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

Lua functionality change: print renamed to mg.write, read renamed to mg.read. Expanded user manual, fixed example pages.