Update to version 1.6.21

Changelog:

• Fix out-of-bound access in create_url_list() (CVE-2016-8863)
• SF Bug Tracker #132 CVE-2016-6255: write files via POST
• If the error or info log files can not be created, use stderr and stdout instead
• miniserver: fix binding to ipv6 link-local addresses

MFH: 2016Q4