/usr/lib/mono/4.5/Mono.Posix.dll is in libmono-posix4.0-cil 4.6.2.7+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/Mono.Posix/4.0.0.0__0738eb9f132ed756/Mono.Posix.dll
/usr/lib/mono/4.5/../gac/Mono.Posix/4.0.0.0__0738eb9f132ed756/Mono.Posix.dll