Security fix

This release fixes a security issue, reported at https://www.cvedetails.com/cve/CVE-2018-10945/

( Fix body length calculation in mg_handle_cgi)

Change List:

• Add mg_url_encode_opt() - a parametrized version of mg_url_encode()

• Add MG_HIDE_SERVER_INFO

• mg_file_upload_handler: Support multiple files

• Add host name verification for OpenSSL

• Add mg_strstrip: trims whitespace at both ends of a string

• Fix http pipelining

• Fix buffer size passed to inet_ntop()

• Fix an edge case in multipart HTTP upload parsing

• Fix mg_http_parse_header (treat ; as a delimiter)

• mg_lwip: Fix race during clean connection teardown

• Fix a bug in mg_mgr_init_opt with opts.num_ifaces

• Fix build with CS_ENABLE_STDIO=0

• Fix warnings on newer versions of GCC

• Fix body length calculation in mg_handle_cgi

• Don't touch send_mbuf when sending MQTT messages