API changes

• Added MG_MK_STR_N(), mg_str_starts_with(), mg_strfree()
• Added mbuf_clear(), mbuf_append_and_free(), mbuf_move()
• Added ability for multipart data handler to provide pushback
• Made MG_CTL_MSG_MESSAGE_SIZE overridable

Bug fixes

• Fixed digest auth result comparison
• Read all available data during recv poll
• Ensure that user sees all the data before connection is closed
• Check for NULL in tcp_recved_tcpip()
• Send extra options with OPTIONS
• Properly support MQTT wildcards when matching topics
• Fix handling of WS handshake error response

Other changes

• Log file:line instead of function; add cs_log_set_file_level()