deploy: Ignore sockets, fifos in /etc during merge

deploy: Ignore sockets, fifos in /etc during merge

https://bugzilla.redhat.com/show_bug.cgi?id=1945274 is an issue where a privileged
kubernetes daemonset is writing a socket into `/etc`.  This makes ostree upgrades barf.

Now, they should clearly move it to `/run`.  However, one option is for us to
just ignore it instead of erroring out.  Some brief investigation shows that
e.g. `git add somesocket` is a silent no-op, which is an argument in favor of ignoring it.

Closes: https://github.com/ostreedev/ostree/issues/2446