core: honor FileDescriptorName= too for Accept=yes sockets

core: honor FileDescriptorName= too for Accept=yes sockets

So far we manually hardcoded $LISTEN_FDNAMES to "varlink" in various
varlink service units we ship, even though FileDescriptorName=varlink
is specified in associated socket units already, because
FileDescriptorName= is currently silently ignored when combined with
Accept=yes. Let's step away from this, which seems saner.

Note that this is technically a compat break, but a mostly negligible
one as there shall be few users setting FileDescriptorName= but
still expecting LISTEN_FDNAMES=connection in the actual executable.

Preparation for #34080