sd-varlink: add flag for sd_varlink_server for creating connections w… (#35841)

…ith fd passing enabled

Let's add a simple flag that enables fd passing for all connections of a
server. It's much easier to use this than to install a connect handler
which manually enables this for each connection.

diff --cc src/mountfsd/mountwork.c
Simple merge

diff --cc src/nsresourced/nsresourcework.c
Simple merge