/usr/lib/mono/2.0/Mono.Posix.dll is in libmono-posix2.0-cil 3.2.8+dfsg-4ubuntu1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/2.0/../gac/Mono.Posix/2.0.0.0__0738eb9f132ed756/Mono.Posix.dll