man-db: TOCTOU bug when processing catman pages

The daily mandb cleanup job in Man-db before 2.7.6.1-1 as packaged in Ubuntu and Debian allows local users with access to the man account to gain privileges via vectors involving insecure chown use.

man-db before 2.8.5 on Gentoo allows local users (with access to the man user account) to gain root privileges because /usr/bin/mandb is executed by root but not owned by root. (Also, the owner can strip the setuid and setgid bits.)