/usr/share/man/man3/sd_is_socket_unix.3.gz is in systemd 44-11+deb7u4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/sd_is_fifo.3.gz
/usr/share/man/man3/sd_is_fifo.3.gz