Update `www/thttpd' to version 2.28, which features:

• Improvements to the FreeBSD startup script (Craig Leres)
• Minor portability tweak in mmc.c
• Fix a potential buffer overrun bug in htpasswd (reported by Alessio Santoru as CVE-2017-17663), no-op on FreeBSD due to our custom patches from Red Hat since r377324

PR: 225701